Image processing apparatus, image processing method, and storage medium for determining dot arrangement for colorants for forming an image on a print medium

ABSTRACT

Provided is quantization processing that can reduce color development defect due to dot overlapping and can output an image with reduced granularity when the image is printed by using multiple kinds of colorants. To this end, dot arrangement information for a colorant for which dot arrangement is already determined among multiple kinds of colorants is acquired for a predetermined region of the image, and an evaluation value of each pixel included in the predetermined region is derived based on the arrangement information. In addition, for the predetermined region, a target value for a predetermined colorant for which dot arrangement is yet to be determined is derived based on the image. Then, whether or not to arrange a dot of the predetermined colorant in the predetermined region is determined based on the target value and the evaluation value.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/704,257, filed on Dec. 5, 2019, which claims the benefit of andpriority to Japanese Patent Application No. 2019-013881, filed Jan. 30,2019 and Japanese Patent Application No. 2018-229551, filed Dec. 7,2018, each of which is hereby incorporated by reference herein in theirentirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to an image processing apparatus, an imageprocessing method, and a storage medium that generate quantized data forforming an image on a print medium.

Description of the Related Art

A dither method is known as a quantization method for printing an imageby using a pseudo gradation method. The dither method uses a dithermatrix in which thresholds associated with individual pixels arearrayed, and determines whether or not to perform dot printing for eachpixel by comparing the associated threshold with a pixel value indicatedby image data.

International Publication No. WO 2002/005545 discloses a dither methodof using a common dither matrix for pieces of image data for multiplekinds of colorants used in a printing apparatus while correcting thedither matrix. Specifically, quantization processing on image data of acolor processed first is performed by directly using thresholds storedin the dither matrix. Then, quantization processing on image data of acolor subsequently processed is performed by using thresholds obtainedby correcting the thresholds stored in the dither matrix based on theimage data of the color processed first.

Japanese Patent Laid-Open No. 2014-739 discloses a quantization methodthat employs the dither method and also provides an upper limit to thenumber in an overlapping manner or size of dots to be printed on thesame pixel and changes, for a pixel exceeding the upper limit, the pixelvalues of its surrounding pixels based on the difference from the upperlimit.

According to International Publication No. WO 2002/005545 and JapanesePatent Laid-Open No. 2014-739, it is possible to reduce the frequency atwhich dots of different colors are printed at the same position on aprint medium in an overlapping manner, thereby reducing colordevelopment defect due to dot overlapping.

However, in the case of International Publication No. WO 2002/005545,when a characteristic pattern is included in image data of a color onwhich the processing is performed first, interference between the imagedata and the threshold matrix affects dot arrangement in a color onwhich the processing is subsequently performed, and degrades thegranularity of the whole image in some cases. In addition, when dots ofdifferent colors are formed in a manner exclusive from each other in aregion, such as an edge part of the image, in which the pixel valueabruptly changes, dots of a color having higher priority are likely toconcentrate at the edge part. As a result, dots of a color having lowerpriority are arranged around the edge part, which causes sharpnessdecrease, local coloring, and the like in some cases.

In the case of Japanese Patent Laid-Open No. 2014-739, it is possible toreduce the frequency at which dots of different colors are arranged atthe same pixel in an overlapping manner, but the difference generated atthe pixel is distributed to its surrounding pixels, and thus dots arehighly likely to be arranged at the surrounding pixels. As a result, thegranularity is lost in some cases.

In sum, according to International Publication No. WO 2002/005545 andJapanese Patent Laid-Open No. 2014-739, it is possible to reduce colordevelopment defect due to dot overlapping, but it is difficult to stablyoutput an image with reduced texture and granularity and excellentsharpness.

SUMMARY OF THE INVENTION

The present invention is intended to solve the above-described problem.Thus, it is an object of the present invention to provide quantizationprocessing that can reduce color development defect due to dotoverlapping and can output an image with reduced granularity andexcellent sharpness when the image is printed by using multiple kinds ofcolorants.

In a first aspect of the present invention, there is provided an imageprocessing apparatus that determines dot arrangement for each of aplurality of kinds of colorants based on an image for the colorant, theimage processing apparatus comprising: an acquisition unit configured toacquire, for a predetermined region of the image, arrangementinformation of dots of a colorant of a first color for which dotarrangement is already determined among the plurality of kinds ofcolorants; a target value derivation unit configured to derive, based ona pixel value in the predetermined region of the image for a secondcolor for which dot arrangement is yet to be determined among theplurality of kinds of colorants, a target value for a colorant of thesecond color in the predetermined region; an evaluation value derivationunit configured to derive an evaluation value for the colorant of thesecond color at each pixel included in the predetermined region based onthe arrangement information; and a dot arrangement determination unitconfigured to determine dot arrangement for the colorant of the secondcolor in the predetermined region based on the target value and theevaluation value.

In a second aspect of the present invention, there is provided an imageprocessing method for determining dot arrangement for each of aplurality of kinds of colorants based on an image for the colorant, theimage processing method comprising: acquiring, for a predeterminedregion of the image, arrangement information of dots of a colorant of afirst color for which dot arrangement is already determined among theplurality of kinds of colorants; deriving, based on a pixel value in thepredetermined region of the image for a second color for which dotarrangement is yet to be determined among the plurality of kinds ofcolorants, a target value for a colorant of the second color in thepredetermined region; deriving an evaluation value for the colorant ofthe second color at each pixel included in the predetermined regionbased on the arrangement information; and determining dot arrangementfor the colorant of the second color in the predetermined region basedon the target value and the evaluation value.

In a third aspect of the present invention, there is provided anon-transitory computer readable storage medium storing a program forcausing a computer to function as each unit of an image processingapparatus that determines dot arrangement for each of a plurality ofkinds of colorants based on an image for the colorant, the imageprocessing apparatus comprising: an acquisition unit configured toacquire, for a predetermined region of the image, arrangementinformation of dots of a colorant of a first color for which dotarrangement is already determined among the plurality of kinds ofcolorants; a target value derivation unit configured to derive, based ona pixel value in the predetermined region of the image for a secondcolor for which dot arrangement is yet to be determined among theplurality of kinds of colorants, a target value for a colorant of thesecond color in the predetermined region; an evaluation value derivationunit configured to derive an evaluation value for the colorant of thesecond color at each pixel included in the predetermined region based onthe arrangement information; and a dot arrangement determination unitconfigured to determine dot arrangement for the colorant of the secondcolor in the predetermined region based on the target value and theevaluation value.

In a fourth aspect of the present invention, there is provided an imageprocessing apparatus that determines dot arrangement for each of aplurality of kinds of colorants by using a threshold matrix based on animage for the colorant, the image processing apparatus comprising: anacquisition unit configured to acquire an output value at an interestpixel for a first color for which dot arrangement is determined amongthe plurality of kinds of colorants; a calculation unit configured tocalculate an exclusion control value for controlling a degree of dotexclusion at the interest pixel for a second color for which dotarrangement is yet to be determined among the plurality of kinds ofcolorants based on the output value at the interest pixel for the firstcolor and a pixel value of the first color at the interest pixel; and adetermination unit configured to determine an output value for thesecond color at the interest pixel based on a pixel value of the secondcolor at the interest pixel, the exclusion control value, and thethreshold matrix.

In a fifth aspect of the present invention, there is provided an imageprocessing method for determining dot arrangement for each of aplurality of kinds of colorants by using a threshold matrix based on animage for the colorant, the image processing method comprising:acquiring an output value at an interest pixel for a first color forwhich dot arrangement is determined among the plurality of kinds ofcolorants; calculating an exclusion control value for controlling adegree of dot exclusion at the interest pixel for a second color forwhich dot arrangement is yet to be determined among the plurality ofkinds of colorants based on the output value for the first color at theinterest pixel and a pixel value of the first color at the interestpixel; and determining an output value for the second color at theinterest pixel based on a pixel value of the second color at theinterest pixel, the exclusion control value, and the threshold matrix.

In a sixth aspect of the present invention, there is provided anon-transitory computer readable storage medium storing a program forcausing a computer to function as each unit of an image processingapparatus that determines dot arrangement for each of a plurality ofkinds of colorants by using a threshold matrix based on an image for thecolorant, the image processing apparatus comprising: an acquisition unitconfigured to acquire an output value at an interest pixel for a firstcolor for which dot arrangement is determined among the plurality ofkinds of colorants; a calculation unit configured to calculate anexclusion control value for controlling a degree of dot exclusion at theinterest pixel for a second color for which dot arrangement is yet to bedetermined among the plurality of kinds of colorants based on the outputvalue at the interest pixel for the first color and a pixel value of thefirst color at the interest pixel; and a determination unit configuredto determine an output value for the second color at the interest pixelbased on a pixel value of the second color at the interest pixel, theexclusion control value, and the threshold matrix.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic views of a printing apparatus and aprinting head;

FIG. 2 is a block diagram for description of a control configuration;

FIG. 3 is a flowchart for description of the process of imageprocessing;

FIG. 4 is a block diagram for description of a software configuration ina first embodiment;

FIG. 5 is a flowchart for description of the process of quantizationprocessing of the first embodiment;

FIG. 6 is a conceptual diagram for description of a processing targetregion setting method;

FIGS. 7A to 7K are diagrams illustrating steps of the quantizationprocessing of the first embodiment;

FIGS. 8A to 8H are diagrams illustrating steps of quantizationprocessing of a second embodiment;

FIG. 9 is a block diagram for description of a software configuration ina third embodiment;

FIG. 10 is a flowchart for description of the process of quantizationprocessing of the third embodiment;

FIGS. 11A to 11J are diagrams illustrating steps of quantizationprocessing for a first color;

FIGS. 12A to 12J are diagrams illustrating steps of quantizationprocessing for a second color;

FIGS. 13A to 13J are diagrams illustrating steps of quantizationprocessing for a third color;

FIGS. 14A to 141 are diagrams illustrating steps of quantizationprocessing for a fourth color;

FIGS. 15A to 15E are comparison diagrams for description of effects ofthe third embodiment;

FIGS. 16A and 16B are comparison diagrams for description of the effectsof the third embodiment;

FIGS. 17A to 17C are comparison diagrams for description of effects ofthe present invention;

FIG. 18 is a block diagram for description of a software configurationin a fourth embodiment;

FIG. 19 is a flowchart for description of the process of quantizationprocessing in the fourth embodiment;

FIG. 20 is a diagram illustrating an exemplary threshold matrix;

FIG. 21 is a diagram illustrating an exemplary processing region;

FIGS. 22A to 22D are diagrams illustrating exemplary gradation data Ik,Ic, and Im;

FIGS. 23A to 23F are diagrams illustrating exemplary K-ink quantizationprocessing;

FIGS. 24A to 24D are diagrams illustrating the process of C-inkquantization;

FIG. 25 is a block diagram for description of a software configurationin a fifth embodiment;

FIG. 26 is a flowchart for description of the process of quantizationprocessing in the fifth embodiment;

FIGS. 27A to 27D are diagrams illustrating a specific example ofquantization processing;

FIGS. 28A to 28I are diagrams illustrating exemplary K-ink quantizationprocessing;

FIGS. 29A to 29I are diagrams illustrating exemplary C-ink quantizationprocessing;

FIGS. 30A to 301 are diagrams illustrating exemplary M-ink quantizationprocessing;

FIGS. 31A to 311 are diagrams illustrating exemplary Y-ink quantizationprocessing; and

FIGS. 32A and 32B are diagrams illustrating inter-color exclusionprocessing and reduction processing results.

DESCRIPTION OF THE EMBODIMENTS

Preferable embodiments of the present invention will be described belowwith reference to the accompanying drawings. Configurations described inthe following embodiments are merely exemplary, and the presentinvention is not necessarily limited to the described configurations.

First Embodiment

FIGS. 1A and 1B are schematic views of a printing apparatus 100 and aprinting head 102 used in the present embodiment. The printing apparatus100 used in the present embodiment is a serial ink-jet printingapparatus. In FIGS. 1A and 1B, an x direction represents the mainscanning direction of the printing head, a y direction represents theconveyance direction of a print medium S, and a z direction representsthe direction of ink discharge.

As illustrated in FIG. 1B, in the printing head 102, nozzle arraysconfigured to discharge ink of cyan (C), magenta (M), yellow (Y), andblack (K), respectively, are arranged in parallel to each other in the xdirection. In each nozzle array, multiple nozzles 101 configured todischarge ink in the z direction in accordance with print data arearrayed in the y direction.

As illustrated in FIG. 1A, the printing head 102 is mounted on acarriage 103, and the carriage 103 is movable in the positive andnegative x directions along a carriage shaft 107. An image of one bandis printed on the print medium S as the printing head 102 discharges inkfrom the nozzles 101 in accordance with print data while the carriage103 moves in the positive and negative x directions. A platen 105 madeof a flat plate is arranged at a position facing a nozzle plane of theprinting head 102. The platen 105 supports, from the back surface, theprint medium S in a region printed by the printing head 102, therebymaintaining a constant distance between the nozzle plane of the printinghead 102 and the print medium S.

When print scanning of one band by the printing head 102 is completed,two pairs of conveyance rollers 104 sandwiching the print medium Srotate to convey the print medium in the positive y direction by adistance corresponding to the above-described one band. As the printscanning of one band by the printing head 102 and the conveyanceoperation of the print medium S by the conveyance rollers 104 asdescribed above are alternately repeated, an image is formed on theprint medium S at stages.

FIG. 2 is a block diagram for description of a control configuration inthe present embodiment. An image processing apparatus 200 is achieved bya host PC or the like, and a CPU 201 executes various kinds ofprocessing by using a RAM 202, which is a transitory storage unit, as awork area in accordance with a computer program held in a HDD 203, whichis a non-transitory storage unit. For example, the CPU 201 generatesprint data printable by the printing apparatus 100 in accordance with acommand received from a user through a keyboard-mouse I/F 205 and acomputer program held in the HDD 203, and outputs the generated printdata to the printing apparatus 100. In addition, information receivedfrom the printing apparatus 100 through a data transmission I/F 204 isdisplayed on a display (not illustrated) through a display I/F 206.

In the printing apparatus 100, a CPU 211 executes various kinds ofprocessing by using a RAM 212, which is a transitory storage unit, as awork area in accordance with a computer program held in a ROM 213, whichis a non-transitory storage unit. Furthermore, the printing apparatus100 includes an image processing accelerator 216 for performing fastimage processing, and a head controller 215 for controlling the printinghead 102.

The image processing accelerator 216 is hardware capable of executingimage processing faster than the CPU 211. The image processingaccelerator 216 is activated by writing parameters and data necessaryfor image processing by the CPU 211 to a predetermined address of theRAM 212, reads the above-described parameters and data, and thenexecutes predetermined image processing on the above-described data.However, the image processing accelerator 216 is not an essentialelement, but the CPU 211 may execute equivalent processing.

The head controller 215 supplies print data to the printing head 102 andcontrols a print operation by the printing head 102. The head controller215 is activated when the CPU 211 writes print data printable by theprinting head 102 and control parameters to a predetermined address ofthe RAM 212, and executes a discharge operation in accordance with theprint data.

A carriage controller 217 controls movement of the carriage 103 in thepositive and negative x directions in accordance with an instructionfrom the CPU 211. In addition, a conveyance controller 218 controlsrotation of the conveyance rollers 104, in other words, conveyance ofthe print medium S in accordance with an instruction from the CPU 211.

USB, IEEE1394, LAN, or the like may be used as a connection scheme atthe data transmission I/F 204 of the image processing apparatus 200 anda data transmission I/F 214 of the printing apparatus 100.

FIG. 3 is a flowchart for description of the process of image processingexecuted by the CPU 201 of the image processing apparatus 200 when aprinting command is generated. The present processing is executed by theCPU 201 by using the RAM 202 as a work area in accordance with acomputer program stored in the HDD 203.

When the present processing is started, the CPU 201 first acquires imagedata produced by an application program or the like, and loads theacquired image data onto the RAM 202. In the present embodiment, imagedata having an 8-bit (256-tone) pixel value indicating luminance of eachof red (R), green (G), and blue (B) is acquired for each pixel.Hereinafter, such image data including a pixel having pixel values ofmultiple components such as R, G, and B is referred to as RGB data byusing the components.

At S301, the CPU 201 executes color correction processing on the imagedata loaded on the RAM 202. The color correction processing isprocessing for converting image data expressing a standard color spacesuch as sRGB into image data associated with color space printable bythe printing apparatus 100. Specifically, the CPU 201 refers to an RGBthree-dimensional look-up table stored in advance and converts RGB dataincluding an 8-bit (256-tone) pixel value into R′G′B′ data including an8-bit (256-tone) pixel value.

At S302, the CPU 201 executes color conversion processing on the R′G′B′data generated at S301. The color conversion processing is processingfor converting R′G′B′ data indicating luminance information into dataindicating density information of K, C, M, and Y for ink colors used bythe printing apparatus 100. Specifically, the CPU 201 refers to thethree-dimensional look-up table stored in advance and converts 8-bit(256-tone) R′G′B′ data of each pixel into 8-bit (256-tone) KCMY data.Data for the ink colors obtained by the color conversion processing iscollectively referred to as KCMY data.

At S303, the CPU 201 performs quantization processing on the KCMY datagenerated at S302. Through the quantization processing, which will bedescribed in detail later, the 8-bit (256-tone) KCMY data is convertedinto 1-bit K′C′M′Y′ data. In the present embodiment, K′C′M′Y′ data isbinary data that specifies dot printing (1) or no dot printing (0) foreach pixel for KCMY ink colors.

At S304, the CPU 201 outputs the K′C′M′Y′ data generated at S303 to theprinting apparatus 100 as print data. Accordingly, the presentprocessing ends.

Having received the print data, the CPU 211 of the printing apparatus100 loads the print data onto the RAM 212. Then, while controlling thehead controller 215, the carriage controller 217, and the conveyancecontroller 218, the CPU 211 prints an image on a print medium inaccordance with the print data loaded on the RAM 212.

FIG. 4 is a block diagram for description of a functional configurationin the quantization processing of the present embodiment. The followingprovides simple description on the function of each block. The KCMY datagenerated through the ink color separation processing at S302 is inputto a region selection unit 401 for each ink color. Hereinafter, data foreach ink color included in the KCMY data is referred to as gradationdata in the present specification. For each gradation data, the numberof pixels is same, and the number of pixels in the longitudinaldirection and the number of pixels in the lateral direction are eachsame. In FIG. 4 , a first color is black (K), a second color is cyan(C), a third color is magenta (M), and a fourth color is yellow (Y).

The region selection unit 401 divides the gradation data includingmultiple pixels into multiple unit regions, and sets one of the unitregions as a processing target region. In this case, the regionselection unit 401 divides the gradation data of each color intomultiple unit regions at the same position. Then, the region selectionunit 401 provides the gradation data of each color for the setprocessing target region to an ink color selection unit 402.

The ink color selection unit 402 refers to quantization orderinformation 408 stored in a memory, selects a processing target colorfrom among the first to fourth colors, and provides the gradation dataof the selected ink color for the processing target region to a targetdot number setting unit 403. The quantization order information 408indicates the order of color for which the quantization processing isexecuted.

The target dot number setting unit 403 refers to a threshold matrix 411stored in the memory, and sets a target number D of dots to be arrangedin the processing target region for each color based on the gradationdata provided by the ink color selection unit 402. The target dot numberis the number of dots to be arranged in the processing target region. Inthis example, through the quantization processing, a binary value thatspecifies dot printing (1) or no dot printing (0) for each pixels. Inother words, the target dot number is the number of pixels for which avalue (in this example, “1”) indicating printing is to be set throughquantization among pixels included in the processing target region.Then, the set target dot number D is provided to a reference valueacquisition unit 404 together with the gradation data for the processingtarget region.

The reference value acquisition unit 404 refers to reference valueinformation 409 stored in the memory and acquires a reference value R ofthe processing target color for the processing target region. In thepresent embodiment, the reference value information 409 provisionallyindicates the priority of dot arrangement on each pixel in theprocessing target region 701 as the reference value.

An evaluation value setting unit 405 refers to exclusion valueinformation 410 stored in the memory, and sets an evaluation value Ev toeach pixel of the processing target color included in the processingtarget region based on the exclusion value information and the referencevalue R acquired by the reference value acquisition unit 404. Theexclusion value is information related to dot arrangement and used forcontrol to avoid dot overlapping. Although a detailed calculation methodwill be described later, it is preferable to dispose no dot on a pixelfor which the exclusion value is larger. In the present embodiment,accumulative dot number information 410 is used as the exclusion valueinformation including the exclusion value of each pixel included in theprocessing target region. The evaluation value Ev indicates a definitivepriority for dot arrangement.

A dot arrangement unit 406 arranges dots on individual pixels includedin the processing target region based on the evaluation value Ev set bythe evaluation value setting unit 405 and the target dot number Ddetermined by the target dot number setting unit. In addition, the dotarrangement unit 406 updates the accumulative dot number information 410based on a result of the arrangement.

A quantized data production unit 407 generates quantized data printableby the printing apparatus 100 based on dot arrangement data generated bythe dot arrangement unit 406, and outputs the generated quantized datato the printing apparatus 100 as print data. The quantized data of eachcolor is equivalent to the K′C′M′Y′ data described with the flowchart inFIG. 3 .

In FIG. 4 , the memory is an integration of the non-transitory HDD 203and the transitory RAM 202 in the image processing apparatus 200described with reference to FIG. 2 . In addition, each block describedabove is a function unit on software executed by the CPU 201 illustratedin FIG. 2 in effect.

FIG. 5 is a flowchart for description of the process of processingexecuted by the CPU 201 by using each block illustrated in FIG. 4 in thequantization processing at S303. In addition, FIGS. 6 and 7A to 7K arediagrams for specific description of steps of the quantizationprocessing of the present embodiment and conversion of the pixel valueof each pixel. The following describes the quantization processingexecuted in the present embodiment in detail in accordance withflowcharts described with reference to FIGS. 5, 6, and 7A to 7K.

When the present processing is started, first at S500, the regionselection unit 401 sets, as a processing target region, one unit regionfrom an image region as a printing target.

FIG. 6 is a conceptual diagram for description of the processing targetregion setting method. The region selection unit 401 divides an imageregion 600 as a printing target into multiple unit regions 601 havingthe same shape, and sets one of the unit regions as a processing targetregion. In the present embodiment, the image region as the printingtarget is partitioned into the units of 4x4 pixels, and each 4x4-pixelregion is treated as a unit region 601. Then, the unit regions 601 aresequentially set as a processing target region 701 and predeterminedquantization processing is performed. FIG. 6 illustrates a situation inwhich the hatched unit region is set as the processing target region701. The order of setting the processing target region is notparticularly limited, but in the present embodiment, the processingtarget region is sequentially set in the positive x direction from theupper-left corner in FIG. 6 , and when processing at the endmost part isended, the processing target region moves onto a row adjacent in thepositive y direction.

The description returns to the flowchart of FIG. 5 . After theprocessing target region is set at S500, the ink color selection unit402 sets a processing target color from among black (first color), cyan(second color), magenta (third color), and yellow (fourth color) atS501. The order of colors to be set as the processing target color isstored in the memory of the HDD 203 as the quantization orderinformation 408 in advance, and the ink color selection unit 402 sets anew color in accordance with the color order indicated by thequantization order information 408 each time the present process isperformed.

At S502, the ink color selection unit 402 loads gradation data of theprocessing target color of the processing target region onto the memory.FIG. 7A illustrates exemplary gradation data loaded at S502. A gradationvalue I of 0 to 255 is associated with each of 4x4 pixels included inthe processing target region 701.

At S503, the target dot number setting unit 403 sets the target dotnumber of the processing target color to be arranged in the processingtarget region. In the present embodiment, the target dot number settingunit 403 sets the target dot number D by referring to the thresholdmatrix 411 stored in the memory in advance.

FIG. 7B illustrates an exemplary threshold matrix 411 referred to atS503. The threshold matrix 411 stores a threshold Th of 0 to 254 inassociation with a pixel position. FIG. 7B only illustrates a pixelregion of 8x12 pixels, but in reality, the pixel position and thethreshold Th are stored in association with each other for a largerpixel region.

The target dot number setting unit 403 first calculates an average valueAve of the gradation values I in the processing target region based onthe gradation data of the processing target color of the processingtarget region. Subsequently, the target dot number setting unit 403reads a matrix (region surrounded by bold lines) of 4x4 (16) thresholdsTh for the processing target region from the threshold matrix 411, andcompares the threshold Th for each pixel with the average value Ave ofthe gradation values I illustrated in FIG. 7A. Then, the number ofpixels for which the average value Ave is larger than the thresholdamong the 16 pixels is counted, and the count value is set as the targetdot number D of the processing target color for the processing targetregion 701.

Specifically, when the gradation data of the processing target region isthat illustrated in FIG. 7A, the average value Ave of the gradationvalue I is calculated to be 1756/16=109.75. Then, the number of pixelsfor which the threshold Th satisfies “Ave>Th” among the 16 pixelssurrounded by bold lines in FIG. 7B is seven. Thus, in this case, thetarget dot number D of the processing target region 701 is seven.

The threshold matrix 411 according to the present embodiment has ablue-noise characteristic weighted on dot dispersity. Specifically, whendots are arranged in the ascending order of the threshold of 0 to 254included in the threshold matrix 411, dot arrangement having excellentdispersity can be stably obtained irrespective of the number of dots.Then, in the threshold matrix 411, the same number of thresholds Thhaving the same value are preferably arranged in a predetermined regionof 16x16 pixels or 256x256 pixels. Specifically, for example, 257 or 256values of 0 to 254 are preferably included in a region of 256x256pixels.

The description returns to the flowchart of FIG. 5 . At S504, thereference value acquisition unit 404 acquires reference valueinformation stored in the memory in advance.

FIGS. 7C to 7E are diagrams illustrating exemplary reference valueinformation 409. In this example, the processing target region 701 has16 pixels of 4×4 pixels, and thus the reference value information isinformation in which a value of 1 to 16 is associated as a provisionalpriority with each of the 16 pixels.

FIG. 7E illustrates the reference value information in the presentembodiment. The reference value acquisition unit 404 reads a 4x4-pixelregion corresponding to the processing target region 701 from thethreshold matrix 411 illustrated in FIG. 7B, and allocates the referencevalue R of 1 to 16 to each pixel in the ascending order of the thresholdTh, thereby producing the reference value information. In this case, thethreshold matrix 411 can be used as the reference value information, andthus a memory capacity prepared for the reference value information canbe reduced. In the present embodiment, the reference value informationis produced based on the threshold matrix, but reference valueinformation prepared in advance may be held. For example, FIG. 7Dillustrates the reference value information held in advance. Similarlyto the threshold matrix used for the quantization processing, thereference value information is of a dot dispersion type. In a case ofthe dot dispersion type, values indicating the priority are arranged indispersion in the reference value information. When the threshold matrixand the reference value information of the dot dispersion type are usedin this manner, dots can be arranged at high dispersity, therebyreducing granularity. In any reference value information illustrated inFIGS. 7C to 7E, the reference value R of 1 to 16 indicates a provisionalvalue of the priority for dot arrangement. Specifically, the provisionalpriority for dot arrangement is highest at a pixel for which thereference value is “1” and is lowest at a pixel for which the referencevalue is “16”.

The description returns to the flowchart of FIG. 5 . At S505, theevaluation value setting unit 405 acquires accumulative dot numberinformation stored in the memory. In the present embodiment, theaccumulative dot number information is information in which theaccumulative number of dots already arranged in the processing targetregion through the quantization processing is stored for each pixelincluded in the processing target region. In other words, when there areother colors for which the quantization processing is executed beforethe processing target color, the accumulative number of dots for theother colors is held as the accumulative dot number information. Thus,when the processing target color is the first color (black), theaccumulative dot number information is set to be “0” for all pixels inthe processing target region.

FIG. 7F illustrates exemplary accumulative dot number information. Inthis example, the current processing target color is the second color(cyan), and a state in which results of quantization of the first color(black) are reflected on the accumulative dot number information isillustrated. Specifically, among the 16 pixels illustrated in FIG. 7F, apixel to which “1” is allocated indicates that one dot is alreadyarranged, and a pixel to which “0” is allocated indicates that no dot isarranged. In the present embodiment, dot arrangement of a color as thenext processing target color is determined by using such accumulativedot number information as the exclusion value information. Hereinafter,the accumulative dot number information is referred to as the exclusionvalue information, and the accumulative dot number of each pixel isreferred to as an exclusion value H.

At S506, the evaluation value setting unit 405 sets the evaluation valueEv for each pixel included in the processing target region based on theexclusion value information acquired at S505 and the reference valueinformation acquired at S504. The evaluation value setting unit 405sets, for each pixel included in the processing target region, theevaluation value Ev so that the evaluation value Ev is larger for apixel for which the exclusion value is smaller and the reference valueis smaller. The evaluation value Ev indicates the priority, and thepriority is higher as the value is smaller.

FIGS. 7G and 7H are diagrams illustrating the procedure of setting theevaluation value Ev based on the reference value information illustratedin FIG. 7E and the exclusion value information illustrated in FIG. 7F.In the present embodiment, arrangement of dots in colors different fromeach other on the same pixel is prevented as much as possible. Thus,when the exclusion value information is that illustrated in FIG. 7F, theevaluation value setting unit 405 first extracts only five pixels forwhich the exclusion value H is “0”, and sets the priority (evaluationvalue) for dot printing among the five pixels based on the referencevalue information illustrated in FIG. 7E. Specifically, the evaluationvalue Ev of 1 to 5 is set to each of the five pixels for which theexclusion value H is “0” in the ascending order of the reference valueR. FIG. 7G illustrates a state in which the evaluation value Ev of 1 to5 is set to each of the five pixels for which the exclusion value H is“0”.

Subsequently, the evaluation value setting unit 405 sets the evaluationvalue Ev of 6 to 16 to each of the remaining pixels for which theexclusion value H is “1” in the ascending order of the reference valueR. FIG. 7H illustrates a state in which the evaluation value Ev of 1 to16 is set to all pixels.

The description returns to the flowchart of FIG. 5 . At S507, the dotarrangement unit 406 arranges dots of the processing target color in theprocessing target region based on the target dot number D set at S503and the evaluation value Ev set at S506. Specifically, dots in a numberindicated by the target dot number D are arranged one by one at eachpixel in the processing target region in the ascending order of theevaluation value Ev set at S506, thereby generating the dot arrangementdata. The dot arrangement unit 406 generates the dot arrangement data ofthe processing target color in the processing target region by storing“1” for each pixel at which a dot is arranged and “0” for a pixel atwhich no dot is arranged.

FIG. 7I is a diagram schematically illustrating dot arrangementdetermined for the processing target region for which the evaluationvalue Ev is set as illustrated in FIG. 7H and the target dot number D isseven. In FIG. 7I, a region illustrated in black represents a pixel atwhich a dot of the processing target color is arranged, and a regionillustrated in white represents a pixel at which no dot of theprocessing target color is arranged. In FIG. 7I, dots are arranged atall pixels for which the exclusion value H is “0” and some of the pixelsfor which the exclusion value H is “1” in FIG. 7F. FIG. 7K illustratesquantized data generated in accordance with the dot arrangement dataillustrated in FIG. 7I. As for the pixels in the processing targetregion, “printing (1)” is associated with each pixel at which a dot ofthe processing target color is arranged, and “no printing (0)” isassociated with the other pixels. Such quantized data indicating“printing (1)” or “no printing (0)” in the processing target region isoutput to the quantized data production unit 407.

At S508, the dot arrangement unit 406 updates the exclusion valueinformation (accumulative dot information) based on the dot arrangementdata generated at S507. Specifically, the exclusion value H for a pixelat which a dot of the processing target color is arranged at S507 isincremented by one and stored in the memory as new exclusion valueinformation (accumulative dot information).

FIG. 7J illustrates a result obtained when the exclusion valueinformation illustrated in FIG. 7F is updated in accordance with the dotarrangement data illustrated in FIG. 7I. In FIG. 7J, a pixel for whichthe exclusion value H is “2” means that a dot of the first color and adot of the second color are arranged at the pixel, and a pixel for whichthe exclusion value H is “1” means that any one of a dot of the firstcolor and a dot of the second color is arranged at the pixel.

The description returns to the flowchart of FIG. 5 . At S509, thequantized data production unit 407 generates quantized data of theprocessing target color in accordance with the dot arrangement datagenerated at S507. In the present embodiment, the quantized dataproduction unit 407 accumulates quantized data of each region to producequantized data of a whole image of the processing target color.

At S510, the region selection unit 401 determines whether thequantization processing for all ink colors is completed in theprocessing target region. When there is an ink color for which thequantization processing is yet to be performed, the process returns toS501 to set the next processing target color and then performs theprocessing at S502 to S509 for the newly set processing target color.When it is determined at S510 that the quantization processing for allink colors is completed in the processing target region, the processproceeds to S511.

At S511, the quantized data production unit 407 determines whether thequantization processing at S502 to S509 is completed for all unitregions. When there is a unit region for which the quantizationprocessing is yet to be performed, the process returns to S500, and theregion selection unit 401 sets the next unit region as a new processingtarget region and performs the processing at S501 to S510 for the newprocessing target region. When it is determined at S511 the quantizationprocessing is completed for all unit regions, the present processing isended.

Through the above-described series of processing, a smaller evaluationvalue Ev is set to a pixel in the processing target region as theexclusion value H is smaller, in other words, as the number of dotsarranged for an already processed ink color is smaller. Then, a dot ofthe processing target color is preferentially arranged at a pixel forwhich the evaluation value Ev is smaller in the processing targetregion. As a result, the number of dots arranged at one pixel in theprocessing target region is reduced, thereby reducing color developmentdefect due to dot overlapping on a print medium and reducinggranularity.

Since dot rearrangement is performed in the processing target regionthat is a unit region including multiple pixels, interference betweenimage data and a threshold matrix can be prevented. Moreover, since dotsin a number (the target dot number) indicated by the gradation data foreach ink color are arranged in each unit region, density and hueindicated by the gradation data are stored in the unit region and can bereproduced on a print medium even when the above-described rearrangementis performed.

Specifically, according to the present embodiment, when an image isprinted by using multiple kinds of colorants, it is possible to outputthe image with reduced color development defect due to dot overlappingand reduced granularity.

In the present embodiment, the quantization processing is executed byusing the threshold matrix having the blue-noise characteristic and thereference value information of the dot dispersion type. However, dotsare concentrated as much as possible in the quantization processing of adot concentration type, and the present embodiment is applicable to thedot concentration type. FIG. 7C illustrates the reference valueinformation of the dot concentration type. In a case of the dotconcentration type, multiple dots are printed in clusters on a printmedium, and thus output concentration is unlikely to be affected by theinter-dot position difference, and the gradation value of each pixel andthe output density can be linearly maintained.

Second Embodiment

In the present embodiment as well, the quantization processing isperformed in accordance with the flowchart illustrated in FIG. 5 byusing the image processing apparatus described with reference to FIGS. 1to 4 . In the present embodiment, the content of the exclusion valueinformation (accumulative dot information) is different from that in thefirst embodiment. In the first embodiment, the exclusion value H beforeupdate is incremented by “1” at a pixel for which dot arrangement of theprocessing target color is determined irrespective of an ink color asthe processing target color. However, in the present embodiment, aweight coefficient wi that is different for each ink color is prepared,and the exclusion value H before update is incremented by the weightcoefficient wi of the processing target color.

In the present embodiment, the weight coefficient of each ink color isset based on an optical density ratio of dots printed on a print medium.In this example, the optical density ratio of 4:2:2:1 is obtained incombination of black, cyan, magenta, and yellow. The weight coefficientof black is “4”, the weight coefficient of cyan is “2”, the weightcoefficient of magenta is “2”, and the weight coefficient of yellow is“1”.

FIGS. 8A to 8H are diagrams illustrating the relation between the dotarrangement data and the exclusion value information in the presentembodiment. The following describes the quantization processing in thepresent embodiment with reference to FIGS. 8A to 8H and in accordancewith the flowchart of FIG. 5 . In the present embodiment as well, atS501, the processing target color is set to be black, cyan, magenta, andyellow in the stated order.

FIG. 8A illustrates exemplary dot arrangement data set at S507 when theprocessing target color is the first color (black). In addition, FIG. 8Billustrates the exclusion value information updated at S508 in thiscase. In FIG. 8B, only for any pixel at which a dot is arranged in FIG.8A, the initial value “0” is incremented by the weight coefficient “4”of black.

FIG. 8C illustrates exemplary dot arrangement data set at S507 when theprocessing target color is the second color (cyan). Cyan dots arearranged based on the exclusion value information illustrated in FIG.8B, and thus the dot arrangement data of cyan has a substantiallyexclusive relation with the dot arrangement data of black illustrated inFIG. 8A.

FIG. 8D illustrates the exclusion value information updated at S508 whenthe processing target color is cyan. In FIG. 8D, only for a pixel atwhich a cyan dot is arranged in FIG. 8C, the exclusion value Hillustrated in FIG. 8B is incremented by the weight coefficient “2” ofcyan.

The following describes a case in which the processing target color isset to be the third color (magenta) in such a state. In this example,the target dot number D is set to be seven at S503, and the referencevalue information illustrated in FIG. 8E is obtained at S504.

In this case, at S506, the CPU 201 refers to the exclusion valueinformation illustrated in FIG. 8D and performs rearrangement of thereference value R illustrated in FIG. 8E. Specifically, first, theevaluation value Ev of 1 to 3 is set to three pixels for which theexclusion value H is “0” in the ascending order of the reference valueR. Subsequently, the evaluation value Ev of 4 to 10 in the ascendingorder of the reference value R is set to seven pixels for which theexclusion value is “2”. Furthermore, the evaluation value Ev of 11 to 15is set to four pixels for which the accumulative dot number is “4” inthe ascending order of the reference value R. Lastly, the evaluationvalue Ev of 16 is set to one pixel for which the accumulative dot numberis “6”. FIG. 8F illustrates evaluation value information obtained whenthe evaluation value Ev of each pixel is set in this manner.

At S507, the dot arrangement unit 406 arranges seven dots indicated bythe target dot number D one by one in the processing target region inthe ascending order of the evaluation value Ev set at S506, therebygenerating the dot arrangement data. FIG. 8G illustrates the dotarrangement data generated in this manner. In the exclusion valueinformation illustrated in FIG. 8D, magenta dot is preferentiallyarranged at any pixel for which the exclusion value H is smaller.

More detailed description is provided. When comparison is performedbetween FIGS. 8A, 8C, and 8G illustrating the dot arrangement data ofblack, cyan, and magenta, a magenta dot is preferentially arranged at apixel at which no black dot nor cyan dot is arranged, and also arrangedat a pixel at which only a cyan dot is arranged. This is because, sincethe weight coefficient “4” of black is larger than the weightcoefficient “2” of cyan, the evaluation value Ev of a pixel at which ablack dot is arranged is larger than the evaluation value Ev of a pixelat which a cyan dot is arranged, and the priority for arrangement of amagenta dot decreases. Then, the priority for arrangement of a magentadot decreases in the order of a pixel at which no dot is arranged, apixel at which only a cyan dot is arranged, a pixel at which only ablack dot is arranged, and a pixel at which a black dot and a cyan dotare arranged.

At S508, the dot arrangement unit 406 updates the exclusion valueinformation based on the dot arrangement data generated at S507.Specifically, the exclusion value H of a pixel at which a dot of theprocessing target color is arranged at S507 is incremented by a magentaweight coefficient w2 of “2” and stored in the memory as new exclusionvalue information. FIG. 8H illustrates the exclusion value informationupdated at S508. The exclusion value illustrated in FIG. 8D isincremented by the magenta weight coefficient w2 of “2” only for a pixelat which a dot is arranged in FIG. 8G. The exclusion value informationillustrated in FIG. 8G is used when the CPU 201 generates the dotarrangement data of the fourth color (yellow) thereafter.

On a print medium, an overlapping dot formed by placing over multipleink colors is easily noticeable and has a decreased color developmentcharacteristic as compared to a single dot formed in one ink color.However, the dot noticeability and the color development characteristicare different between overlapping dots due to combinations of ink colorsforming the overlapping dots. For example, an overlapping dot formed ofcyan and yellow is unlikely to be noticeable and has small influence ongranularity as compared to an overlapping dot formed of black and cyan.When the exclusion value H is managed by using the weight coefficientdifferent for each ink color and a dot of the processing target color ispreferentially arranged at a pixel for which the exclusion value H issmall as in the present embodiment, the granularity can be reduced whiledegradation of the color development characteristic is prevented as muchas possible in a situation in which printing of an overlapping dotcannot be avoided.

In the above description, the weight coefficient of each color is setbased on the optical density ratio of dots printed on a print medium,but the present embodiment is not limited thereto. For example,saturation decrease when a dot is overlapped among special inks such asgreen, orange, and purple has larger influence on an image thansaturation decrease when a dot is overlapped among the above-describedfour color inks. Thus, in such a case, the weight coefficient of eachink color is preferably set based on not only the optical density butalso another factor such as the hue or the saturation.

With a particular ink color combination such as a combination of blackand gray or a combination of cyan and light cyan, the hue and thesaturation are not largely affected by dot overlapping. Thus, in such acase, multiple pieces of the exclusion value information may be preparedin association with ink colors, and the weight coefficient when theexclusion value H is updated may be different between the pieces of theexclusion value information. Then, when each ink color is the processingtarget color, at S505, the associated exclusion value information may beacquired from among the multiple pieces of the exclusion valueinformation.

For example, the weight coefficient of black in the exclusion valueinformation for gray is preferably set to be smaller than the weightcoefficient of black in the exclusion value information for cyan. Inaddition, the weight coefficient of cyan in the exclusion valueinformation for light cyan is preferably set to be smaller than theweight coefficient of cyan in the exclusion value information formagenta. In this manner, overlapping of ink colors between which the hueis similar is prioritized to overlapping of ink colors between the hueis different, and the color development characteristic of the wholeimage can be improved.

In addition, the weight coefficient as described above may be changedfor each processing target region. For example, when the hue of theprocessing target region indicated by R′G′B′ data obtained at S301 inFIG. 3 is close to G (green), overlapping between a cyan dot and ayellow dot is preferably prioritized in some cases. In such a case, inthe exclusion value information for yellow, the weight coefficient ofcyan needs to be set to be smaller than the weight coefficients of blackand magenta.

According to the present embodiment described above, when an image isprinted by using multiple colorants, the image can be output withreduced color development defect due to dot overlapping and with reducedgranularity.

Third Embodiment

In the present embodiment as well, a series of image processing isperformed in accordance with the flowchart illustrated in FIG. 3 byusing the image processing apparatus described with reference to FIGS. 1and 2 .

FIG. 9 is a block diagram for description of a functional configurationin the quantization processing of the present embodiment. Differencesfrom FIG. 4 are as follows: an evaluation value setting unit 901 usesthe threshold matrix 411 as the reference value information, and a dotarrangement history information 903 is prepared in place of theaccumulative dot number information 410. The dot arrangement historyinformation 903 is information in which dot arrangement information ofan ink color for which the dot arrangement data is already generated isstored in association with the ink color. The evaluation value settingunit 901 according to the present embodiment sets the evaluation valueEv for each pixel by using the threshold matrix 411 and the dotarrangement history information 903. In addition, a dot arrangement unit902 according to the present embodiment updates the dot arrangementhistory information 903 based on the set dot arrangement data.

FIG. 10 is a flowchart for description of the process of processingexecuted by the CPU 201 according to the present embodiment by usingeach block illustrated in FIG. 9 in the quantization processing at S303.In addition, FIGS. 11A to 11J are diagrams for specific description ofsteps of the quantization processing when the processing target color isthe first color (black). The following describes the quantizationprocessing for the first color (black) in accordance with a flowchartillustrated in FIG. 10 with reference to FIGS. 11A to 11J.

At S1002, the ink color selection unit 402 loads the gradation data ofblack of the processing target region onto the memory. In this example,the gradation data illustrated in FIG. 11A is loaded. At S1003, thetarget dot number setting unit 403 sets the target number D of blackdots to be arranged in the processing target region. Specifically,first, 4x4 thresholds for the processing target region are read from thethreshold matrix 411 and each compared with the average value Ave of thegradation value of the gradation data illustrated in FIG. 11A. Then, thenumber of pixels for which the average value Ave is larger than thethreshold among the 16 pixels is counted, and the count value is set asthe target dot number D. When the gradation data is that illustrated inFIG. 11A, the average value Ave is 128. In addition, when the 4x4thresholds read in association with the processing target region arethose illustrated in FIG. 11C, the target dot number D is set to beeight.

At S1004, the evaluation value setting unit 901 acquires and loads thereference value information stored in the memory in advance. In thisexample, the 4x4 thresholds illustrated in FIG. 11C, which are also usedat S1003, are used as the reference value information.

At S1005, the evaluation value setting unit 901 acquires the dotarrangement history information 903 (refer to FIG. 9 ) stored in thememory. In the present embodiment, the dot arrangement historyinformation is the dot arrangement data of each ink color for which thequantization processing is ended in the processing target region. Whenthe processing target color is the first color (black), the CPU 201acquires null data. When the processing target color is the second color(cyan), the CPU 201 acquires the dot arrangement history information ofblack. When the processing target color is the third color (magenta),the CPU 201 acquires the dot arrangement history information of blackand the dot arrangement history information of cyan.

At S1006, the evaluation value setting unit 901 derives and sets theevaluation value Ev for each pixel included in the processing targetregion based on the dot arrangement history information acquired atS1005 and the reference value information (FIG. 11C) acquired at S1004.The following describes an evaluation value deriving method in thepresent embodiment in detail.

To calculate the evaluation value Ev, the evaluation value setting unit901 according to the present embodiment first calculates a firstevaluation value Ev1 for each pixel included in the processing targetregion. The first evaluation value Ev1 is calculated in accordance withExpression (1).E1=(I/Imax)−(R/Rmax)−(H/Hmax)  Expression (1)

In the expression, I represents the gradation value of an interest pixelindicated by the gradation data, and Imax represents the maximum valuethereof. In the present embodiment, the gradation data is of 8 bits (256tones), and thus Imax is 255. For example, when the gradation data is of16 bits (65536 tones), Imax is 65535. FIG. 11B illustrates normalizedvalues I/Imax for 4×4 pixels for the processing target region. Eachvalue I/Imax is a real number having a value of 0 to 1, and a dot ismore likely to be arranged at a pixel for which the value is larger.

In the expression, R represents the reference value of each pixelindicated by the reference value information, and Rmax represents themaximum value thereof. In a case of the present embodiment, since thereference value information is obtained from the threshold matrix 411,Rmax is 254. FIG. 11D illustrates normalized values R/Rmax of thereference value information. Each value R/Rmax is a real number having avalue of 0 to 1, and a larger value means that the provisional priorityfor dot arrangement is lower at a pixel.

In the expression, H represents the exclusion value obtained from thedot arrangement history information. In the present embodiment, theexclusion value H can be obtained by using Expression (2).H=Σ(wi×Di)  Expression (2)

In the expression, i represents a number indicating the processing orderof the processing target color. In the present embodiment, since thequantization processing is performed in the order of black, cyan,magenta, and yellow, i is 1, 2, 3, and 4 when the processing target inkis black, cyan, magenta, and yellow, respectively. In addition, wirepresents the weight coefficient for each ink color. In the presentembodiment, the weight coefficient of each ink color is set based on theoptical density ratio of dots printed on a print medium so that w1 isfour, w2 is two, w3 is two, and w4 is one.

In Expression (2), Di represents the number of dots of an ink color iarranged at each pixel. For example, when a black dot is arranged at apixel, D1 is one at the pixel. Furthermore, E represents a sum totalover 1 to (i-1).

In this manner, the exclusion value H is calculated by calculating theproduct of the number Di of dots arranged at each pixel and the weightcoefficient wi specific to each ink color i and summing the products ofall the ink colors i processed before the processing target color. Whenthe processing target color is the first color (black), in other words,in a case of i=1, the exclusion value H is zero for all pixels. Thus,the exclusion value information of 4x4 pixels as illustrated in FIG. 11Eis obtained by using Expression (2).

In Expression (1), Hmax represents the maximum value of the exclusionvalue H in the processing target region. In the present embodiment, thevalue H/Hmax obtained by normalizing the exclusion value H with themaximum value Hmax is referred to as an exclusion control value Hn. Theexclusion control value Hn is a real number having a value of 0 to 1,and a larger value indicates that a dot is more unlikely to be arrangedat a pixel. When the processing target color is the first color (black),the maximum value Hmax is zero, but the exclusion control value Hn isfixed to zero. FIG. 11F illustrates exclusion control value informationin the processing target region when the processing target color is thefirst color (black).

FIG. 11G is a diagram illustrating first evaluation value informationcalculated in accordance with Expression (1) based on the gradation data(I/Imax) after normalization in FIG. 11B, the reference valueinformation (R/Rmax) after normalization in FIG. 11D, and the exclusioncontrol value information (Hn) in FIG. 11F. In the present embodiment,the first evaluation value Ev1 indicates the easiness of dot arrangementat each pixel. A dot is more likely to be arranged at a pixel for whichthe value is positively larger, and a dot is more unlikely to bearranged at a pixel for which the value is negatively larger.

Subsequently, the evaluation value setting unit 901 sets a value of 1 to16 as a second evaluation value Ev2 for each pixel in the processingtarget region in the descending order of the first evaluation value Ev1.When there are multiple pixels for which the first evaluation value Ev1is same, a pixel having a larger gradation value I may be prioritized, apixel positioned on the upper-left side in the processing target regionmay be prioritized, or, the priority may be set at random.

FIG. 11H illustrates the evaluation value information in which thesecond evaluation value Ev2 of 1 to 16 is set for each pixel included inthe processing target region. Hereinafter, the second evaluation valueEv2 is simply referred to as the evaluation value Ev in the presentembodiment.

The description returns to the flowchart of FIG. 10 . At S1007, the dotarrangement unit 902 arranges a dot of the processing target color ateach pixel included in the processing target region based on the targetdot number D set at S1003 and the evaluation value Ev set at S1006.Specifically, dots in a number indicated by the target dot number D arearranged one by one in the processing target region in the ascendingorder of the evaluation value Ev set at S1006, thereby generating thedot arrangement data.

FIG. 11I is a diagram illustrating the dot arrangement data generatedfor the processing target region for which the target dot number D iseight through the dot arrangement determination processing at S1007based on the evaluation value information in FIG. 11H. In FIG. 11I, aregion illustrated in black represents a pixel at which a black dot isarranged, and a region illustrated in white represents a pixel at whichno black dot is arranged.

At S1008, the dot arrangement unit 902 updates the dot arrangementhistory information based on the dot arrangement data generated atS1007. Specifically, the dot arrangement data generated at S1007 isstored in the memory in association with the ink color i. In the presentexample, the dot arrangement data illustrated in FIG. 11I is stored inthe memory in association with the first color (black). The stored dotarrangement history information is read from the memory as the dotarrangement history information of the first color (black) at S1005after the processing target color is changed.

At S1009, the quantized data production unit 407 generates quantizeddata in accordance with the dot arrangement data generated at S1007, andstores the generated quantized data in the memory. FIG. 11I illustratesquantized data generated in accordance with the dot arrangement dataillustrated in FIG. 11I.

When the above-described processing for black is completed, the processreturns to S1001, the second color (cyan) is set as the processingtarget color, and the processing at S1002 to S1009 is performed.

FIGS. 12A to 12J are diagrams illustrating steps of processing when theprocessing target color is the second color (cyan), similarly to FIGS.11A to 11J. FIG. 12A illustrates the gradation data of cyan loaded atS1002. In this case, the gradation data has normalized values I/Imax asillustrated in FIG. 12B.

FIG. 12C illustrates 4x4 thresholds for the processing target region andread from the threshold matrix for cyan, and based on this information,the target dot number D of cyan is set to be four at S1003. In addition,the 4x4 thresholds illustrated in FIG. 12C are used as reference valueinformation R, and the reference value information R has normalizedvalues R/Rmax as illustrated in FIG. 12D.

FIG. 12E illustrates the exclusion value information derived when theprocessing target color is cyan. The exclusion value information isderived by referring to the dot arrangement history information(information same as the quantized data in FIG. 11I) of the first color(black) and calculating the exclusion value H of each pixel inaccordance with Expression (2). In a case of the exclusion valueinformation illustrated in FIG. 12E, the maximum value Hmax of theexclusion value H is four, and the exclusion control value informationas illustrated in FIG. 12F is obtained by normalizing the exclusionvalue H of each pixel.

FIG. 12G is a diagram illustrating the first evaluation valueinformation derived in accordance with Expression (1) based on thegradation data (I/Imax) after normalization illustrated in FIG. 12B, thereference value information (R/Rmax) after normalization illustrated inFIG. 12D, and exclusion value control value information illustrated inFIG. 12F. In addition, FIG. 12H illustrates the evaluation valueinformation for cyan derived based on the exclusion value control valueinformation in FIG. 12G. Furthermore, FIG. 12I illustrates the dotarrangement data set for the processing target region for which theevaluation value Ev is set as illustrated in FIG. 12H and the target dotnumber D is four, and FIG. 12J illustrates the quantized data based onthe data in FIG. 12I. In the dot arrangement data illustrated in FIG.12I, dots are arranged at positions in an exclusive relation with thosein the dot arrangement data of the first color (black) illustrated inFIG. 11I.

Thereafter, processing of the third color (magenta) and processing ofthe fourth color (yellow) are performed for the processing target regionin the stated order in accordance with the above-described process.Details of the processing are same as those in the cases of the firstcolor (black) and the second color (cyan), and description thereof isomitted. FIGS. 13A to 13J are diagrams illustrating steps of theprocessing when the processing target color is the third color(magenta), similarly to FIGS. 11A to 11J. FIGS. 14A to 14J are diagramsillustrating steps of the processing when the processing target color isthe fourth color (yellow), similarly to FIGS. 11A to 11J.

According to the present embodiment described above, the evaluationvalue Ev indicating the priority for dot arrangement is calculated foreach pixel based on Expression (1) including the three parameters of thegradation value I, the reference value R, and the exclusion value H.Thus, the gradation value I of each pixel can be more actively reflectedon a quantization result of the pixel than in the above-describedembodiments in which the evaluation value Ev is set with the twoparameters of the reference value R and the exclusion value H.

For example, when a picture image is printed, it is preferable thatuniformity of the whole image is preferred to the gradation value ofeach pixel and dots are dispersed in a unit region without overlappingas in the first and second embodiments. However, when a graphic imageincluding characters and line drawing is printed, it may be preferablethat the gradation value of each pixel is reflected on dot existence atthe pixel as much as possible to prefer the sharpness and contrast of anobject. When dots are arranged based on Expression (1) including thethree parameters of the gradation value I, the reference value R, andthe exclusion value H as in the present embodiment, the gradation valueof each pixel can be reflected on dot existence at the pixel while dotoverlapping between different colors is prevented. As a result, an imageexcellent in the sharpness and the color development characteristic canbe output.

The balance between the sharpness and the color developmentcharacteristic may be adjusted by multiplying I/Imax, which prompts dotarrangement, and H/Hmax, which prevents dot arrangement, by weightcoefficients Wp and Wh having a value of 1 to 0, respectively. In thiscase, for example, when the weight coefficient Wp is increased, animage, the sharpness and the contrast of which are maintained, can beoutput. When the weight coefficient Wh is increased, the ratio ofexclusion between dots in different colors is increased, and an imageexcellent in the color development characteristic can be output.

In addition, when a weight coefficient Wr for the normalized valueR/Rmax of the reference value R is prepared, the influence of anarrangement order defined by the threshold matrix can be increased byincreasing the value of the weight coefficient. For example, when aprepared threshold matrix has the blue-noise characteristic withexcellent dispersity, dot arrangement excellent in dispersity can beobtained. Such weight coefficients Wp, Wr, and Wh may be changed asappropriate in accordance with the ink color, the printing mode, thekind of a print medium, and the like.

According to the present embodiment, since the evaluation value Ev isderived based on the gradation value I in addition to the referencevalue R and the exclusion value H, the frequency of overlappingarrangement of dots of different colorants is higher than in the firstand second embodiments. This may increase the granularity as compared tothe first and second embodiments but increases the resistance(robustness) of the image quality against various error and fluctuationof the printing apparatus. This will be briefly described below.

For example, when the dot arrangement data is generated so that a dot offirst ink and a dot of second ink are exclusive from each other but aprinting positional shift occurs between a printing head for the firstink and a printing head for the second ink, some overlapping dots aregenerated on a print medium. When the amount of such printing positionalshift varies with reciprocate scanning of the printing heads andcockling of the print medium, the number of generated overlapping dotsvaries with the printing positional shift amount as well, and theoverlapping dots are recognized as density unevenness and colorunevenness on the print medium.

However, when overlapping dots of different colorants are allowed togenerate at a predetermined ratio in advance as in the presentembodiment, two dots that should be printed at different positionsoverlap with each other at some places, but two dots that should overlapwith each other are separated from each other at some places. Thus, whenthe printing positional shift amount varies with reciprocate scanning ofthe printing heads and cockling of the print medium, the ratio ofoverlapping dots is maintained in a constant range so that densityunevenness and color unevenness are unlikely to be recognized on theprint medium. Such image robustness against the printing positionalshift can be adjusted by changing the above-described weight coefficientWh.

The following describes effects of the present embodiment with referenceto specific examples. For example, when eight dots are arranged inaccordance with the reference value information illustrated in FIG. 11Cwithout calculating the evaluation value Ev as in the presentembodiment, the dot arrangement data as illustrated in FIG. 15A isobtained. In comparison of FIG. 15A with FIG. 11I as the dot arrangementdata in the present embodiment, characteristics of the gradation dataillustrated in FIG. 11A more clearly appear in FIG. 11I, whichillustrates the dot arrangement data in the present embodiment, than inFIG. 15A. Thus, according to the present embodiment, the gradation valueI is included in Expression (1) for calculating the evaluation value Ev,and thus characteristics of the gradation data can be more stronglyreflected on a quantization result.

The following describes effects of the exclusion value H included inExpression (1) for deriving the evaluation value Ev. For example, FIG.16A is obtained when no term of the exclusion value H is provided inExpression (1) and the first evaluation value information is calculatedin accordance with the gradation data of the second color (cyan)illustrated in FIG. 12A and the reference value information illustratedin FIG. 12C. When four dots are arranged in accordance with thisevaluation value information, the dot arrangement data as illustrated inFIG. 16B is obtained.

When four dots are arranged in accordance with the reference valueinformation illustrated in FIG. 12C, the dot arrangement data asillustrated in FIG. 16B is obtained. In comparison of FIG. 16B with FIG.11I, which is the dot arrangement data in the present embodiment, FIG.16B is less excellent in exclusiveness from the dot arrangement data ofthe first color (black) illustrated in FIG. 11I than FIG. 11I.

Specifically, according to the present embodiment, the exclusion value Hincluded in Expression (1) for deriving the evaluation value Ev canincrease the exclusiveness of the dot arrangement data of the processingtarget color from the dot arrangement data of ink for which thequantization processing is already completed.

In Expression (1), the first evaluation value Ev1 is calculated byperforming addition and/or subtraction on a value obtained bynormalizing each of the gradation value I, the reference value R, andthe exclusion value H to the range of 0 to 1, but such normalizationprocessing is not essential. The gradation value I, the reference valueR, and the exclusion value H only need to be adjusted to the same range,and may be each, for example, an integer of 1 to 256 expressed in 8 bitsor an integer of 1 to 65536 expressed in 16 bits.

When a region of the threshold matrix corresponding to the processingtarget region is used as the reference value information as in thepresent embodiment, the content of the reference value information isdifferent between unit regions. Consider a case in which the target dotnumber D as in the present embodiment is not set and the reference valueinformation is used as a dither matrix when dither processing isperformed on the gradation data illustrated in FIG. 11A. In this case,the dot arrangement data illustrated in FIG. 11I can be obtained fromthe reference value information illustrated in FIG. 11C, and the dotarrangement data illustrated in FIG. 15C can be obtained from thereference value information illustrated in FIG. 15B. Accordingly, withthe dot arrangement data quantized in accordance with the same gradationdata illustrated in FIG. 11A, eight dots are arranged in a unit regionand 11 dots are arranged in another unit region. As a result, occurrenceof density unevenness and color unevenness on a print medium isconcerned.

However, when the average value of gradation values included in a unitregion is calculated and the number of dots to be arranged in the unitregion is set based on the average value as in the present embodiment,fluctuation in the number of arranged dots between unit regions can bestabilized to some extent irrespective of variation in the content ofthe reference value information. FIGS. 15D and 15E illustrate the firstevaluation value information and the dot arrangement data, respectively,when processing of the present embodiment is executed based on thegradation data in FIG. 11A by using the reference value informationillustrated in FIG. 15B. In FIG. 15E, nine dots are arranged to expressdensity substantially equivalent to that in FIG. 11I in which eight dotsare arranged.

In the present embodiment, a different threshold matrix is prepared foreach of the four ink colors to calculate the target dot number D and theevaluation value Ev as illustrated in FIGS. 11 to 14 , but the thresholdmatrix may be common to the colors. In this case, the exclusion valueinformation as the sum total of the dot arrangement data of alreadyprocessed ink colors is information based on the threshold matrix commonto all colors, and as a result, dispersity based on the threshold matrixcan be obtained as the sum total over all ink colors. Thus, whendispersity increase is preferred, in particular, for example, onethreshold matrix having the blue-noise characteristic may be preparedand used as the reference value information common to the colors. Inthis manner, a smooth image having high dispersity as the sum total overall ink colors can be output.

However, in the present embodiment, the threshold matrix does notnecessarily need to be used as the reference value information. As inthe first embodiment, the reference value information may be prepared asan integer of 1 to 16 (or 0 to 15) separately from the threshold matrix.In this case, the maximum value Rmax used for normalization of thereference value is 16 (or 15).

FIGS. 17A to 17C are diagrams for comparison of the present inventionwith a case in which the quantization processing is performed by amethod of International Publication No. WO 2002/005545. As similar toFIG. 12A, FIG. 17A illustrates the gradation data of the second color(cyan) in the processing target region. FIG. 17B illustrates a thresholdmatrix for cyan data obtained through shift processing in accordancewith the method of International Publication No. WO 2002/005545.

FIG. 17B illustrates a matrix obtained by subtracting each gradationvalue I indicated by the gradation data in FIG. 11A from thecorresponding threshold in the matrix illustrated in FIG. 11C andincrementing the resulting value by 255 only when the value is negative.Then, when dither processing is performed in accordance with thegradation data in FIG. 17A by using the threshold matrix in FIG. 17Bobtained in this manner, dot arrangement data as illustrated in FIG. 17Cis obtained. In comparison of the dot arrangement data illustrated inFIG. 17C with the dot arrangement data of the second color (cyan)according to the present embodiment illustrated in FIG. 12I,exclusiveness from the dot arrangement data of the first colorillustrated in FIG. 11I is less excellent in FIG. 17C than in FIG. 12I.

According to the method of International Publication No. WO 2002/005545,a dot of the first color and a dot of the second color can be arrangedin a completely exclusive manner when the gradation value of the firstcolor for which the quantization processing is performed beforehand isuniform and the gradation value of the second color for which thequantization processing is subsequently performed is uniform. However,in an actual image, the gradation value often varies between pixels tosome extent as illustrated in FIG. 11A, and it is difficult to printdots of the first color and the second color in a stable exclusionrelation by the method of International Publication No. WO 2002/005545.

However, in the present embodiment, unless the sum of the number of dotsof the first color and the second color exceeds the number of pixels(16) in a unit region, a dot of the first color and a dot of the secondcolor can be arranged in the unit region in a completely exclusiverelation as illustrated in FIGS. 11I and 12I. Such an exclusion relationcan be maintained for any unit region. Accordingly, according to thepresent embodiment, the frequency that dots of different colors areprinted in duplication at the same position on a print medium can befurther reduced as compared to International Publication No. WO2002/005545, and an image having a higher color developmentcharacteristic can be output.

In the above, FIGS. 11 and 12 used in the present embodiment arecompared with FIGS. 17A to 17C of International Publication No. WO2002/005545 and described as effects of the present embodiment, but sucheffects can be also obtained in the first and second embodiments.

Modifications of First to Third Embodiments

Although the above-described embodiments describe the case in which thegradation value I is converted into binary data of “printing (1)” or “noprinting (0)”, the present invention is also applicable to quantizationof the gradation value into three values or more. When the number ofquantized values is three or more, the quantized values of each pixelcan be expressed by adjusting the number and size of dots printed in anarea on a print medium corresponding to one pixel.

The following specifically describes an example in which the number ofquantized values is three. In this case, Imax is first adjusted toobtain a normalized value I/Imax of 0 to 2 from the gradation value I.Then, one dot is first arranged at a pixel for which the normalizedvalue I/Imax is larger than one. In addition, a value obtained bysubtracting one from the normalized value I/Imax of the pixel is set asa new normalized value of the pixel.

Thereafter, the evaluation value Ev is calculated based on the newnormalized value by the method described above in the embodimentstogether with a pixel at which a dot is yet to be arranged, and a dot isarranged again. Then, the quantized value of a pixel at which two dotsare arranged as a result is set to be “2”, and two dots are printed inthe corresponding area on the print medium. In addition, the quantizedvalue of a pixel at which one dot is arranged as a result is set to be“1”, and one dot is printed in the corresponding area on the printmedium. Furthermore, the quantized value of a pixel at which no dot isarranged as a result is set to be “0”, and no dot is printed in thecorresponding area on the print medium. In this case, a quantized valueof 0 to 2 may be stored in the accumulative dot information in the firstand second embodiments and the dot arrangement history information inthe third embodiment.

To print two dots in an area corresponding to one pixel, for example,two nozzle arrays in each of which the multiple nozzles 101 are arrayedmay be prepared in the x direction for each ink color at the printinghead 102 illustrated in FIG. 1B. In addition, a multipath printingmethod in which print scanning is performed multiple times on anidentical image region of the print medium by the printing head 102 maybe employed. Furthermore, binary data at higher resolution may begenerated by providing index loading processing to the quantized valueof three values or more and may be used as definitive print data.

When the quantized value of three values or more is expressed not in thenumber of dots but in the dot size, a large dot may be arranged at apixel for which the quantized value is “2”, a small dot may be arrangedat a pixel for which the quantized value is “1”, and no dot may bearranged at a pixel for which the quantized value is “0”. In this case,the target dot number set at S503 in FIG. 5 or S1003 in FIG. 10 is atarget value for the amount of ink of the processing target color to beapplied in an area on the print medium corresponding to the processingtarget region, which is integrated for the number of dots or the dotsize. The target dot number setting unit 403 in FIG. 4 is a target valuederivation unit for deriving and setting such a target value.

In the above description, the target dot number D in the processingtarget region is set by comparing the average value of gradation valuesin the gradation data with each threshold in the threshold matrix, butthe target dot number setting method is not limited thereto. Forexample, the target dot number D can be derived in accordance withExpression (3) by using a sum SUM of multiple gradation values I in theprocessing target region and a natural number N (1≤N≤255) set at random.D=0when SUM<ND=INT((SUM−N)/255)+1when SUM≥N  Expression (3)

In the expression, INT(x) is a function that returns a natural numbernot exceeding x. For example, SUM is 1756 in the gradation data in FIG.7A described in the first embodiment. Calculation of the target dotnumber by using Expression (3) with N=128 obtains:D=((1756−128)/255)+1=7since SUM≥N. In this example, N is the center value “128” of thegradation value I, but N is preferably set at random within a range notlargely different from the center value. In this manner, the target dotnumber for the processing target region can be obtained without storinga threshold matrix having a large size in the memory.

In the above description, the first, second, third, and fourth colorsare set as black, cyan, magenta, and yellow, respectively, in accordancewith the order of colors stored in the quantization order information408, but the present invention is not limited to this configuration.However, for an ink color at a later order, the reference valueinformation is more unlikely to be reflected on the evaluation valueinformation than the exclusion value, and thus dispersity tends to belost. For this reason, in the present embodiment, processing isperformed in the descending order of the optical density of dots printedon a print medium, and yellow, with which the optical density is lowest,is applied as the fourth color for which dispersity is most likely to belost.

However, the color order of processing may be changed for each unitregion. Particularly when the reference value information of adispersion type is used, repetitive pattern and texture are visuallyrecognized in some cases due to processing performed in the same colororder for all unit regions. In such a case, the pattern and texture canbe made unnoticeable across the whole image by switching the color orderof processing for each unit region. For example, the first to thirdcolors may be sequentially switched between black, cyan, and magentawhile yellow, which is visually unnoticeable, is fixed as the fourthcolor.

In addition, the color order of processing may be set based on imagedata. For example, when the hue of the processing target regionindicated by the R′G′B′ data obtained at S301 in FIG. 3 is close to G(green), the first color may be set to be cyan, the second color may beset to be yellow, the third color may be set to be black, and the fourthcolor may be set to be magenta.

In the flowcharts described with reference to FIGS. 5 and 10 , theprocessing target region is set, and then the processing target color isset for the set processing target region, but the order of thesesettings may be opposite. Specifically, the processing target color maybe first set, the quantization processing for the set processing targetcolor may be performed for all unit regions, and thereafter, theprocessing target color may be switched to the next ink color. However,in this case, a memory region for storing the accumulative dot numberinformation and the dot arrangement history information needs to beprepared for all unit regions. Thus, the order in accordance with eachof the flowcharts described with reference to FIGS. 5 and 10 is morepreferable for saving of such a memory region.

In the above description, the reference value information for theprocessing target region is acquired based on the threshold matrix andthe reference value information stored in the memory in advance, but thereference value information may be generated based on the gradation dataor the like when the quantization processing is performed. For example,the gradation data in the processing target region may be referred to,and the reference value of 1 to 16 may be sequentially allocated to apixel for which the gradation value is largest. In addition, after thegradation data is provided with predetermined edge processing andfiltering processing, the reference value of 1 to 16 may be sequentiallyallocated to a pixel for example the gradation value is largest. In thismanner, the reference value R is generated in accordance with parameterssuch as image characteristics (edge and ink amount) and then exclusionprocessing is performed based on the reference value R, and thus colordevelopment defect due to dot overlapping can be reduced while the imagecharacteristics are appropriately reproduced.

In the above description, 4×4 pixels are associated with a unit regionby referring to FIG. 6 , but the unit region according to the presentinvention is not limited to such a size and a shape. The unit region mayhave a larger size, and the number of pixels in the x direction may notbe equal to the number of pixels in the y direction. With a smallernumber of pixels included in the unit region, the memory capacity usedfor processing can be reduced and the gradation value I and the imagecharacteristics are more likely to appear on an output image. With alarger number of pixels per unit region, exclusiveness in the unitregion and thus the color development characteristic can be improved toreduce the granularity.

However, when the size of the unit region is n×m pixels, the numbers ofpixels in the x direction and the y direction included in image data arepreferably integer multiples of n and m. Thus, when such a condition isnot satisfied, white data is preferably appended in predeterminedregions at end parts of the image data in the x direction and the ydirection so that the numbers of pixels in the x direction and the ydirection become integer multiples of n and m.

Fourth Embodiment

In the present embodiment as well, the image processing apparatusdescribed with reference to FIGS. 1 to 3 is used. FIG. 18 is a blockdiagram for description of a functional configuration in quantizationprocessing of a fourth embodiment. The following provides simpledescription on the function of each block. In the present embodiment,the KCMY data generated through the ink color separation processing atS302 is input to a pixel selection unit 1801 for each ink color. In FIG.18 , the first color is black (K), the second color is cyan (C), thethird color is magenta (M), and the fourth color is yellow (Y).

The pixel selection unit 1801 sets, as an interest pixel, one pixel fromthe gradation data including multiple pixels. Then, the gradation valueof each color at the set interest pixel is provided to an ink colorselection unit 1802. In other words, in this example, having set theinterest pixel, the pixel selection unit 1801 outputs the gradationvalue for each of the four colors at the interest pixel to the ink colorselection unit 1802.

The ink color selection unit 1802 refers to quantization orderinformation 1808 stored in the memory, selects the processing targetcolor from among the first to fourth colors, and provides the gradationdata for the selected ink color in the processing target region to athreshold processing unit 1803.

The threshold processing unit 1803 executes quantization processing foreach pixel by using a threshold matrix 1809 stored in the memory inadvance. FIG. 20 is a diagram illustrating part of the threshold matrix1809 used in the present embodiment. In the threshold matrix 1809,multiple thresholds having values different from each other are arrangedat random in a manner associated with the pixels. When the thresholdmatrix 1809 is associated in a tile form with a processing target image,each pixel in the image is associated with any of the thresholds. Thethreshold processing unit 1803 uses the same threshold matrix 1809 forthe gradation data of all the C, M, Y, K colors in association with thesame positions.

An exclusion processing unit 1804 controls thresholds for a color to beprocessed next so that overlapping of dots of different colors at theinterest pixel is prevented as much as possible. Thus, the exclusionprocessing unit 1804 updates the threshold at the interest pixel eachtime the threshold processing unit 1803 quantizes the gradation value atthe interest pixel.

A contrast calculation unit 1805 calculates a contrast value incorrelation with the degree of contrast in an adjacent region includingthe interest pixel. The contrast is high when the gradation valuedifference is large in the adjacent region, and the contrast is low whenthe gradation value difference is not so large. An exclusion controlunit 1806 controls the degree of an exclusion effect by the exclusionprocessing unit 1804.

A quantized data production unit 1807 generates quantized data printableby the printing apparatus 100 based on the dot arrangement datagenerated by the threshold processing unit 1803, and outputs thegenerated quantized data to the printing apparatus 100 as print data.The quantized data of colors is equivalent to the K′C′M′Y′ datadescribed with reference to the flowchart in FIG. 3 .

The following description is made based on an assumption that a singledot size is applied and 1-bit quantized data indicating “ON/OFF” of adot for each pixel is generated for each of CMYK.

FIG. 19 is a flowchart for description of the process of processingexecuted by the CPU 201 by using each block illustrated in FIG. 18 inthe quantization processing at S303. The following describes, in detailwith reference to FIGS. 20,21, and 22A to 22D, the quantizationprocessing executed in the present embodiment in accordance with theflowchart illustrated in FIG. 19 .

When the present processing is started, first at S1901, the pixelselection unit 1801 selects, as an interest pixel, a processing targetpixel (x, y) from an image as a printing target.

Subsequently at S1902, the ink color selection unit 1802 sets aprocessing target color from among black (first color), cyan (the secondcolor), magenta (the third color), and yellow (fourth color). The orderof colors set as the processing target color is stored in advance in thememory of the HDD 203 as the quantization order information 1808, andthe ink color selection unit 1802 sets a new color in accordance withthe color order indicated by the quantization order information 408 eachtime the present process is performed. In this example, the order ofquantization is K→C→M→Y.

At S1903, the threshold processing unit 1803 acquires the gradationvalue I (x, y) of the processing target color for the interest pixel inthe gradation data obtained by the color separation processing (stepS302 in FIG. 3 ). In the present embodiment, the gradation data is 8-bitdata in which a gradation value of 0 to 255 is stored for each pixel.

At S1904, the threshold processing unit 1803 acquires a threshold thused for the quantization processing of the interest pixel. Thethreshold for the interest pixel (x, y) is referred to as the thresholdth (x, y).

Subsequently at S1905, the threshold processing unit 1803 determines anoutput value indicating “ON” and “OFF” of a dot at the interest pixelthrough magnitude comparison between the gradation value I (x, y) andthe threshold th (x, y). More specifically, in a case of I (x, y)>th (x,y), the process proceeds to S1907, and the threshold processing unit1803 sets “ON” (output value=1) to the dot at the interest pixel (x, y)at S1907. In a case of I (x, y) th (x, y), the process proceeds toS1906, and the threshold processing unit 1803 sets “OFF” (outputvalue=0) to the dot at the interest pixel (x, y) at S1906.

Subsequently at S1908, the exclusion processing unit 1804 changes thethreshold th (x, y) for the interest pixel, which is used for the nextprocessing target color. More specifically, the exclusion processingunit 1804 updates the threshold th (x, y) to a threshold th′ (x, y) sothat a dot of another color is unlikely to be arranged at the interestpixel (x, y) for which “dot ON” is set at S1907. In the presentembodiment, the exclusion processing unit 1804 updates the threshold byusing Expression (4).th′=th(x,y)−I(x,y)  Expression(4)

When the threshold th′ thus calculated is incremented by the maximumvalue “255” of the gradation data and used for the quantizationprocessing, the threshold th′ is larger than the original threshold thfor a pixel at which a dot is arranged, and thus another dot is unlikelyto be arranged at the pixel in the subsequent processing, therebyachieving the exclusion effect. The exclusion processing unit 1804stores the new threshold th (x, y) in the memory. The exclusivearrangement of dots of different colors at the interest pixel throughsuch threshold update will be described later.

Subsequently at step S1909, the contrast calculation unit 1805calculates accumulative gradation data S for each pixel included in apredetermined region centered at the interest pixel. For example, theinterest pixel is a pixel 2101 in an input image 2100 as illustrated inFIG. 21 . In this case, the contrast calculation unit 1805 sets a3x3-pixel block centered at the interest pixel 2101 to be a processingregion. For example, when a pixel is positioned at an edge of the imagelike a pixel 2102 and the processing region of 3x3 pixels cannot beobtained, the processing region is set to be a smaller block of 2x2 orthe like. In the present embodiment, the contrast calculation unit 1805calculates, as the accumulative gradation data S, the sum total of thegradation value of a color processed at the interest pixel and thegradation value I of the processing target color.

For example, the interest target color at the interest pixel is K. Inthe present embodiment, since the order of quantization is K→C→M→Y, nocolor is processed at the interest pixel, and the accumulative gradationdata S of each pixel in the processing region is a gradation value Ikfor K ink. When the interest target color is C ink, the sum of thegradation value Ik of K ink as a color processed at the interest pixeland a gradation value Ic of C ink as the processing target color iscalculated as the accumulative gradation data S for each pixel in theprocessing region. Similarly, when the processing target color is M inkat the interest pixel, the sum of the gradation value Ik, the gradationvalue Ic, and a gradation value Im is calculated as the accumulativegradation data S for each pixel in the processing region. In addition,when the processing target color is Y ink at the interest pixel, the sumtotal of the gradation values Ik, Ic, Im, and Iy is calculated as theaccumulative gradation data for each pixel in the processing region.FIGS. 22A to 22C illustrate exemplary gradation values Ik, Ic, and Im ofK, C, and M inks for each pixel in the processing region. When theprocessing target color is M ink at the interest pixel, the accumulativegradation data S of each pixel calculated at S1909 is a valueillustrated in FIG. 22D.

Subsequently at step S1910, the contrast calculation unit 1805calculates a contrast value cnt indicating contrast in the processingregion based on the accumulative gradation data S of each pixel in theprocessing region. Specifically, in the present embodiment, the contrastvalue cnt is calculated by using Expression (5) below. The maximum andminimum values among the accumulative gradation data S of the pixels inthe processing region are represented by max_S and min_S, respectively.cnt=(max_S−min_S)/(max_S+min_S)  Expression (5)

However, cnt is zero when max_S is zero. The contrast value cntcalculated by using Expression (5) is a value of 0 to 1 and larger forhigher contrast in the processing region.

At S1911, the exclusion control unit 1806 performs processing ofreducing the strength of exclusion processing for the interest pixelbased on the contrast cnt in the processing region for the interestpixel. More specifically, the threshold th (x, y) for the interest pixelis further updated so that the threshold change at S1908 is canceled asthe contrast value cnt in the processing region is larger (contrast ishigher). In the present embodiment, the exclusion control unit 1806calculates a new threshold th″ by using Expression (6) below and storesthe threshold th″ in the memory as a threshold for the interest pixel.th″=th′(x,y)+I(x,y)×cnt  Expression (6)

In Expression (6), “I (x, y)×cnt” functions as an exclusion controlvalue for controlling the degree of exclusion. Specifically, thethreshold th′ for exclusively arranging a dot is maintained for a pixelhaving a small contrast value (low contrast) in an adjacent region. Thethreshold th′ is updated for a pixel having a large contrast value(large contrast) in the adjacent region. Particularly when the contrastvalue of the interest pixel is one, the threshold is updated to thevalue of the original threshold th. This processing will be describedlater in detail.

Subsequently at S1912, the exclusion control unit 1806 determineswhether the threshold th″ (x, y) is smaller than zero. When thethreshold th″ (x, y) is smaller than zero, the process proceeds toS1913. At S1913, the exclusion control unit 1806 updates the thresholdof the interest pixel by setting, as a new threshold th″′, a valueobtained by incrementing the threshold th″ (x, y) by an upper limitvalue i_max (in this example, 255) in the gradation data. When thethreshold th″ (x, y) is equal to or larger than zero, the process skipsS1913 and proceeds to S1914.

Subsequently at S1914, the ink color selection unit 1802 determineswhether the quantization processing for all colors at the interest pixelis executed in accordance with a quantization order information 1808.When the quantization processing for all colors at the interest pixel iscompleted, the process proceeds to S1915. When there is a color yet tobe processed, the process proceeds to S1902 to select the processingtarget color at the interest pixel and continue the quantizationprocessing at the interest pixel.

At S1915, the quantized data production unit 1807 outputs the quantizedvalue of each color of the interest pixel to the RAM 212. At S1916, thepixel selection unit 1801 determines whether the quantization processingis executed for all pixels of an input image. For example, when pixelsare selected sequentially from the upper-left corner, whether thequantized data of a pixel closest to the lower-right corner is producedmay be determined. As a result of the determination, when thequantization processing is executed for all pixels of the input image,the quantization processing is ended. When there is a pixel for whichthe quantized data is yet to be produced, the process returns to S1901to newly select the interest pixel and continue the quantizationprocessing.

The following describes a specific example of the quantizationprocessing in the present embodiment with reference to FIGS. 23A to 23Fand 24A to 24D. FIG. 23A illustrates the gradation data of K ink for apartial region in an input image. Each rectangle represents one pixel,and a gradation value is stored in each pixel. FIG. 23B illustratesthresholds for the partial region illustrated in FIG. 23A. FIG. 23Cillustrates a result of K-ink quantization processing. In FIG. 23C, eachpixel colored in black indicates that a dot of K ink (hereinafter alsosimply referred to as a K dot) is arranged at the pixel (the outputvalue is “1”). Since K ink is the first color, the output value is “1”for each pixel for which the gradation value is larger than thecorresponding threshold. When the K-ink quantization processing isexecuted, at least some of the thresholds for the pixels are updatedthrough the processing at S1908 to S1914. FIG. 23D illustrates thethreshold th′ set by Expression (4) at S1908. For each pixel, a valueobtained by subtracting the gradation value from the threshold is storedas the threshold th′. The output value of “1” means that the gradationvalue of the pixel is larger than the threshold. Thus, the threshold th′is negative for a pixel for which the output value is “1”.

FIG. 23E illustrates the contrast value cnt of each pixel calculated atS1910. However, the processing region for which the contrast value iscalculated is a 3x3-pixel region centered at the interest pixel. Thecontrast value is small (0.00) for a pixel that is flat (has no pixelvalue change) and has a low contrast in an adjacent region. The contrastvalue is larger for a pixel having higher contrast in the adjacentregion.

FIG. 23F illustrates the threshold th″ calculated based on the exclusioncontrol value for controlling the strength of the exclusion processing.However, th″ is incremented by i_max=255 when th″ is smaller than zero.As a result of the exclusion control, for a pixel, such as thelower-right pixel in the partial region, at which a K dot is arrangedand the contrast is low, a value larger than the original threshold “19”is employed as a threshold. In addition, for a pixel, such as the twolowest pixels on the second column, at which K dot is arranged and thecontrast is high, a value smaller than the original threshold isemployed to reduce the strength of the exclusion processing. Inaddition, for a pixel, such as the upper-right pixel or the pixel on thesecond column and the second row, at which no K dot is arranged, a valuesmaller than the original threshold is employed.

Subsequently, C ink is selected as the processing target color. FIG. 24Aillustrates the gradation data of C ink in a partial region same as thepartial region illustrated in FIG. 23A.

In the present embodiment, each gradation value of C ink illustrated inFIG. 24A is quantized by using not the thresholds illustrated in FIG.23B but thresholds after the exclusion control illustrated in FIG. 23F.FIG. 24D illustrates a result of the quantization in the presentembodiment. For description of the effects of the present embodiment,FIG. 24B illustrates a result of quantization by using the thresholdsillustrated in FIG. 23B. FIG. 24C illustrates a result of quantizationby using the threshold th′ after the exclusion processing. In FIGS. 10Bto 10C, a dot of C ink (hereinafter referred to as a C dot) is arrangedat each pixel colored in black.

As illustrated in FIGS. 23C and 24B, when the gradation value of eachcolor is quantized by using the same thresholds, a C dot is formed ateach pixel at which a K dot is formed. In other words, the rate ofoverlapping between dots of different colors is large when identicalthresholds are used for different ink colors. When characteristics ofthe gradation data are similar like a line of one pixel width on thesecond column from the left in each of FIGS. 23A and 24A, in particular,the rate of dot overlapping is likely to be high through thequantization by using the same thresholds. However, when a printingapparatus 100 discharges ink onto a print medium based on the quantizeddata, dot overlapping and paper white exposure both occur in a flatregion. As a result, the contrast is high despite of the flat region,and the granularity degrades due to overlapping dots, and accordingly,the image quality is lost in some cases. In the partial regionillustrated in FIG. 24B, the region is flat on the three right columns,and image degradation due to dot overlapping potentially occurs. Inaddition, paper white increases due to increase of the number ofoverlapping dots, and desired density cannot be obtained in some cases.

Quantization data illustrated in FIG. 24C is obtained when the gradationvalue of each pixel is quantized by using the threshold th′ after theexclusion processing illustrated in FIG. 23D. For the exclusion effectas described above, th′ is incremented by i_max=255 to perform thequantization when the threshold th′ is smaller than zero. As illustratedin FIGS. 23C and 24C, when the threshold th′ after the exclusionprocessing is used, no C dot is formed at a pixel at which a K dot isformed. In other words, dots of different colors are exclusivelyarranged due to the threshold change at S1908. Through such exclusivearrangement of dots of different colors, degradation of the granularitycan be reduced in a flat region.

However, it is not preferable in some cases to exclusively arrange dotsof different colors in a region in which the contrast is high. Forexample, in the gradation data illustrated in FIGS. 23A and 24A,gradation values on the second column from the left are larger thangradation values on the other columns, and accordingly, a line of K andC is formed. However, when the printing apparatus 100 discharges inkonto a print medium based on the C-ink quantized data illustrated inFIG. 24C, there is no dot at which K ink and C ink overlap with eachother in a region in which a high-density line due to K and Coverlapping should be formed. As a result, K and C dots are arranged indispersion, and thus the density is flat across the partial region. As aresult, a line of K and C is unlikely to be perceived on the printmedium.

Thus, in the present embodiment, the processing of controlling thedegree of exclusion between dots of different colors for each pixelbased on the contrast at S1911 is performed. FIG. 24D illustrates, asthresholds after the exclusion control, C-ink quantized data obtained byusing the threshold matrix after the exclusion control illustrated inFIG. 23F. As illustrated in FIGS. 23C and 24D, the line of K and C isreproduced with dots overlapped near the line of K and C along which thecontrast is high, but C and K dots are exclusively arranged in a uniformregion on the right side.

In this manner, in the present embodiment, the inter-color exclusionprocessing and the strength of the exclusion processing are controlled.The control is performed so that dots of different colors are likely tooverlap with each other in a region in which the contrast is high, anddots of different colors are unlikely to overlap with each other in aregion in which the contrast is low. Accordingly, an image at highquality can be achieved when dots of different colors are appropriatelyprinted in an overlapping manner on the print medium.

The processing region for which the contrast value cnt is calculated isnot limited to 3x3 pixels. The interest pixel may not be positioned atthe center, and the contrast cnt may be calculated for a 4x4 region withthe interest pixel positioned at the upper-left corner. Alternatively,the processing region may not be a continuous region but may be astaggered lattice or a processing region in which calculation isperformed on pixels on odd-numbered or even-numbered columns. Inaddition, another method other than the calculation method by usingExpression (5) may be applied as the method of calculating the contrastvalue cnt. For example, the contrast value cnt may be calculated basedon a histogram of the accumulative gradation data S. Specifically, akurtosis may be calculated from the histogram in a predetermined regioncentered at the interest pixel, and the contrast cnt may be set to behigher as the kurtosis is smaller.

Fifth Embodiment

The fourth embodiment describes the method of controlling the degree ofinter-color dot exclusion in a method in which an identical thresholdmatrix is used for the gradation data of each color. The presentembodiment describes a method of controlling the degree of exclusionbetween different colors when different threshold matrices are used bycalculating the number of dots by using an average value in apredetermined region and then determining arrangement of the number ofdots. Any component same as that in the fourth embodiment will bedescribed by using the same name, and detailed description thereof willbe omitted.

FIG. 25 is a block diagram for description of a functional configurationin the quantization processing of the present embodiment. A regionselection unit 2501 divides the gradation data including multiple pixelsinto multiple unit regions, and sets one of the unit regions as aprocessing target region. In this case, the region selection unit 2501divides the gradation data of each color into multiple unit regions atthe same positions.

An ink color selection unit 2502 determines a processing target inkcolor for each region.

A threshold matrix 2510 for each ink color is stored in the memory. Dothistory information 2511 stores output values of pixels of each color ina unit region. Thus, the dot history information 2511 indicates, foreach pixel in the processing target region, dot existence for each inkcolor determined beforehand. In the present embodiment, the number ofink colors is four, and thus the dot history information 2511 is capableof storing output values of the unit region for three colors.

A target dot number setting unit 2503 sets, for each color, the targetnumber D of dots to be arranged in the processing target region. Thetarget dot number is defined as in the first embodiment.

An evaluation value setting unit 2504 sets an evaluation value H foreach pixel included in the processing target region of the processingtarget color based on the gradation data of the processing targetregion, thresholds for the processing target region, and the dot historyinformation. A detailed calculation method will be described later. Adot arrangement unit 2507 determines dot arrangement in the processingtarget region based on the target dot number and the evaluation value ofeach pixel. A quantized data production unit 2508 produces and outputsquantized data of an image.

FIG. 26 is a flowchart for description of the process of processingexecuted by the CPU 201 by using each block illustrated in FIG. 25 inthe quantization processing at S303.

First at S2600, the region selection unit 2501 sets, as a processingtarget region, one unit region from an image region as a printingtarget. Definition and an installation method of the processing targetregion are same as those in the first embodiment described withreference to FIG. 6 , and thus description thereof is omitted.

When the processing target region is set at S2600, the ink colorselection unit 2502 sets a processing target color from among black(first color), cyan (the second color), magenta (the third color), andyellow (fourth color) based on quantization order information 2509 atS2601.

Subsequently at S2602, the target dot number setting unit 2503 loads,onto the memory, the gradation data of the processing target color ofthe processing target region among the gradation data obtained throughthe ink color separation processing (S302 in FIG. 3 ). FIG. 27Aillustrates exemplary gradation data loaded at S2602. The gradationvalue I of 0 to 255 is associated with each of 4x4 pixels included inthe processing target region 701. At S2603, the target dot numbersetting unit 2503 acquires the threshold th (x, y) for each of the 16pixels included in the processing target region from the thresholdmatrix 2510 of the processing target color. FIG. 27B is a diagramillustrating part of the threshold matrix of the processing targetcolor. The target dot number setting unit 2503 reads, from the memory,16 thresholds at positions corresponding to the processing target regionin the threshold matrix 2510.

Subsequently at S2604, the target dot number setting unit 2503 sets,based on the gradation data acquired at S1202 and the threshold th (x,y) acquired at S1203, the number of dots to be arranged in theprocessing target region. Specifically, an average value obtained bydividing the sum total SUM of the gradation data in the processingregion by the pixel number “16” is compared in magnitude with eachthreshold in the processing target region, and the number of pixels forwhich the average value is larger than the threshold is set as thenumber of dots. In the present embodiment, the threshold th is acquiredfrom the threshold matrix different for each processing target color.For example, in a case of the processing target region illustrated inFIG. 27A, the target dot number setting unit 2503 first calculates theaverage value “109.75” (=1756/16) of gradation values in the processingtarget region. Subsequently, the average value is compared with eachthreshold, and the number of pixels for which the average value islarger than the threshold, the number being seven, is calculated as thenumber of dots in the processing target region.

Subsequently at S2605, an evaluation value setting unit 2504 calculatesan evaluation value E for evaluating the priority for dot arrangementfor each of the 16 pixels in the processing target region. Theevaluation value E is a real number indicating that a dot is arranged inpriority as the value is larger. Specifically, the evaluation valuesetting unit 2504 calculates the evaluation value E of each pixel basedon Expression (7) below.E=I/I_max−th/th_max  Expression (7)

In the expression, I represents the gradation value of the interestpixel, and I_max represents the upper limit value of the gradation data.For example, I_max is 255 when the gradation data is of 8 bits, andI_max is 65535 when the gradation data is of 16 bits. Accordingly,I/I_max is a real number of 0 to 1, and a dot is more likely to bearranged as the value is larger. In addition, th represents a thresholdfor the interest pixel, and th_max represents the upper limit value ofthe threshold. For example, th_max may be 254 when the threshold is 0 to254. In this case, th/th_max is a real number of 0 to 1, and a dot ismore unlikely to be arranged as the value is larger.

Subsequently at S2606, an exclusion processing unit 2505 acquires, foreach of the 16 pixels in the processing target region, a dotaccumulation value at the pixel from dot history information 1111 storedin the memory.

For example, when the processing target color is C, the dot historyinformation 2511 stores output values of K ink for which processing isalready performed. Similarly, when the processing target color is M, thedot history information 2511 stores the sum of output values of K inkand output values of C ink, or when the processing target color is Y,the dot history information 2511 stores the sum of output values of K,C, and M ink. When the processing target color is K ink, there is no inkcolor for which processing is already performed, and thus the exclusionprocessing unit 2505 acquires nothing from the dot history information2511.

Subsequently at S2607, the exclusion processing unit 2505 corrects theevaluation value E of each pixel based on the dot history information2511. Specifically, the exclusion processing unit 2505 calculates anevaluation value E′ after the exclusion processing by using Expressions(8) and (9).E′=E−H/H_max  Expression (8)where H=Σ(wi×Di)  Expression (9)

In Expression (8), H represents the exclusion control value calculatedfrom the history information, and is a real number indicating that a dotis more unlikely to be arranged as the value is larger. In the presentembodiment, the exclusion control value H is larger for a pixel at whicha larger number of dots are arranged at higher density.

In Expression (9) above, i represents a number indicating the processingorder of the processing target color. For example, since the processingorder is K→C→M→Y in the present embodiment, i is one when the processingtarget color is K, and i is two when the processing target color is C.In addition, wi represents a weight set in advance for the i-th inkcolor. In the present embodiment, the weight is set to be larger for anink color having a higher density. Specifically, the weights for the inkcolors satisfy K:C:M:Y=4:2:2:1. In other words, w1 is “4”, w2 is “2”, w3is “2”, and w4 is “1”. When the weights are determined in this manner,the exclusion control value H is calculated to be larger as dots arearranged at a higher density.

In addition, Di represents the number of dots of the i-th ink color. Forexample, when a dot of K ink is arranged at the interest pixel, D1 is“1”. In addition, E represents a sum total up to (i-1). However, H is“0” when i is “1”. The exclusion control value H calculated as describedabove is calculated in accordance with the number of already arrangeddots and the density of arranged dots. The value is larger when thenumber of dots is larger or the density of arranged dots is higher.

In addition, H_max represents the maximum value of the exclusion controlvalue H in the processing region. The exclusion control value H in theprocessing region is normalized to the range of 0 to 1 by dividing theexclusion control value H by H_max. In Expression (8), E is set to be Ewhen H_max is “0”. In other words, when the processing target color isthe first color, no dot is arranged yet, and thus the evaluation value Eis not corrected.

Subsequently at S2608, an exclusion control unit 2506 corrects, based onthe accumulative gradation data S at each pixel, the evaluation value Eagain to cancel the effect of correction of the evaluation value at stepS2607. Specifically, the exclusion control unit 2506 calculates anevaluation value E″ after the exclusion control for each pixel by usingExpression (10) below.E″=E′+S/S_max  Expression (10)where S=Σ(wi×Ii)  Expression (11)

In the expression, the accumulative gradation data S is a weighted sumof the gradation value I of an ink color for which processing is alreadyperformed. For example, when the processing target color is K, theaccumulative gradation data S of each pixel is zero. In addition, whenthe processing target color is C ink, the product of the gradation valueIk of K ink and the weight w1 for K ink is acquired as the accumulativegradation data S for each pixel. Similarly, when the processing targetcolor is M ink, “Ik×w1+Ic×w2” is acquired for each pixel.

In Expression (10), S_max represents the upper limit value of theaccumulative gradation data S in theory. For example, when w1 is “4” andthe gradation data is 8-bit data having a value of 0 to 255, the valueof S_max at i=2 is 1020 (=255×4). In addition, when w2 is “2”, the valueof S_max at i=3 is 1530 (=255×4+255×2). When S_max is “0”, E″ is set tobe E′.

In the present embodiment, the weight wi in Expressions (9) and (10) issame for all inks, but different weights may be used for identical inkin each expression.

Subsequently at S2609, the dot arrangement unit 2507 determines dotarrangement in the processing target region based on the evaluationvalue E″ of each pixel in the processing target region. Specifically,dots in the target dot number calculated at step S2604 are arranged inthe region sequentially for each pixel in the descending order of theevaluation value E″. When the evaluation value E″ is same betweenpixels, does are arranged in priority at a pixel for which the gradationvalue I is larger. As a result, dot arrangement is determined for aprocessing unit region as illustrated in FIG. 27C. Alternatively, apixel in priority may be selected at random from among pixels betweenthe evaluation value is same, or a pixel closer to the upper-left cornermay be arranged in priority. A dot arrangement unit 2507 sets “1” to theoutput value of a pixel at which a dot is arranged in the processingtarget region, and sets “0” to the output value of a pixel at which nodot is arranged. FIG. 27D is a diagram illustrating the output value ofeach pixel.

Subsequently at S2610, the dot arrangement unit 2507 updates the dothistory information 2511. Specifically, the output value of each pixelset at S2609 is stored on the memory in association with an ink color.At S2606 for the next ink color, the stored dot history information 2511is acquired as the dot history of an ink color for which processing isalready performed.

Subsequently at S2611, a quantized data production unit 2508 producesimage quantized data from the output value of each unit region set atS2609, and outputs the produced quantized data to the RAM 212.

Subsequently at S2612, the ink color selection unit 2502 determineswhether the quantized data of all ink colors is produced for theprocessing target region in accordance with the order of quantization.When it is determined that the quantized data of all ink colors isproduced, the process proceeds to S2613. When there is ink for which thequantized data is yet to be produced, the process proceeds to S2601 toselect the processing target color again and continue the quantizationprocessing.

At S2613, the region selection unit 2501 determines whether thequantized data is produced for all pixels of an input image.Specifically, it is determined whether each divided block of the 4x4pixels illustrated in FIG. 6 is selected as the processing target regionand the quantized data is produced. For example, when the blocks areselected sequentially from the upper-left corner, it may be determinedwhether the processing region is a block closest to the lower-rightcorner. As a result of the determination, when the quantized data isproduced for all pixels of the input image, the quantization processingis ended. When there is a pixel for which the quantized data is yet tobe produced, the process proceeds to S2600 to select the processingregion and continue the quantization processing.

The following specifically describes the quantization processing in thepresent embodiment with reference to FIGS. 28A to 28I and 29A to 29I.The description is first made on the quantization processing for K inkas the first color with reference to FIGS. 28A to 28I.

FIG. 28A illustrates the gradation data Ik of K ink in the processingtarget region. FIG. 28B illustrates thresholds for the processing targetregion.

In this case, at S2604, the target dot number is calculated to be “4”through comparison of an average value 60 of the gradation data Ikillustrated in FIG. 28A with each threshold illustrated in FIG. 28B.

FIG. 28C illustrates the evaluation value E calculated by usingExpression (6) at S2605. FIG. 28D illustrates the exclusion controlvalue H before normalization. Since K ink is the first color (w=1), H is“0” for all pixels in the region. Thus, the evaluation value E after theexclusion processing at S2607 is E as illustrated in FIG. 28E.

Similarly, the accumulative gradation data S illustrated in FIG. 28F is“0” for all pixels in the region, and the evaluation value E″ after theexclusion control processing illustrated in FIG. 28G is E. In this case,dot arrangement of K ink illustrated in FIG. 28H can be obtained bysequentially arranging four dots (dot number=4) at a pixel in thedescending order of the evaluation value E″ illustrated in FIG. 28G. Inaddition, the quantized data illustrated in FIG. 28I is stored on thememory as the dot history information 2511 in association with theweight w1=4.

FIGS. 29A to 29I illustrate the process of the quantization processingfor C ink as the second color. FIGS. 29A and 29B illustrate gradationdata Ic of C ink in the processing target region and thresholds for theprocessing target region, respectively. In this case, the evaluationvalue E illustrated in FIG. 29C and the number of dots=4 are calculated.

In addition, the exclusion control value H of each pixel illustrated inFIG. 29D is calculated from the quantized data of K ink illustrated inFIG. 28I and the weight w1=4 for K ink. In this case, H_max is “4”.

In addition, the evaluation value E after the exclusion processingillustrated in FIG. 29E is calculated from the evaluation value Eillustrated in FIG. 29C and the exclusion control value H normalizationwith H_max.

Similarly, the accumulative gradation data S of each pixel illustratedin FIG. 29F is calculated from the gradation data of K ink illustratedin FIG. 28A and the weight w1=4 for K ink. S_max is “1020” for w1=4. Inaddition, the evaluation value E″ after the exclusion control processingillustrated in FIG. 29G is calculated from the evaluation value E′ afterthe exclusion processing and the accumulative gradation data Snormalized with S_max.

In this case, dot arrangement of C ink illustrated in FIG. 29H can beobtained by sequentially arranging four dots (dot number=4) at a pixelin the descending order of the evaluation value E″ illustrated in FIG.29G. In addition, the quantized data illustrated in FIG. 29I is storedon the memory as the dot history information 2511 in association withthe weight w2=2 for C ink.

Similarly, FIGS. 30A to 301 illustrate the process of the quantizationprocessing for M ink as the third color. In this example, H_max is “6”,S_max is “1530”, and the number of dots=“4”. Thus, the quantized data ofM ink illustrated in FIG. 301 can be obtained.

In addition, FIGS. 31A to 311 illustrate the process of the quantizationprocessing for Y ink as the fourth color. The weight w3 for M ink is“2”. In this example, H_max is “8”, S_max is “2040”, and the number ofdots is “4”. Thus, the quantized data for Y ink illustrated in FIG. 311can be obtained. Through the above-described processing, the quantizeddata for the four colors of C, M, K, and Y can be obtained.

The following describes effects of the exclusion control in the presentembodiment. When the processing target color is the second color or alater color 2, the exclusion control value H is a value based on alreadydetermined dot arrangement. For example, when the processing targetcolor is C ink, the exclusion control value H is positive for a pixel atwhich a K dot is arranged, and a dot is unlikely to be arranged at thepixel. Specifically, FIG. 32A illustrates dot arrangement when dots inthe dot number=4 are arranged based on the evaluation value E before theexclusion processing illustrated in FIG. 29C. Similarly, FIG. 32Billustrates dot arrangement when dots in the dot number=4 are arrangedbased on the evaluation value E′ after the exclusion processingillustrated in FIG. 29E. As illustrated in FIG. 32A, in the dotarrangement based on the evaluation value E before the exclusionprocessing, a dot is arranged at a pixel at which K ink is arranged, andthus the arrangement is not exclusive between colors. However, asillustrated in FIG. 32B, in the dot arrangement based on the evaluationvalue E after the exclusion processing, a dot is arranged at a pixelother than a pixel at which K ink is arranged. In other words, thearrangement exclusive between different colors is obtained. In thismanner, the dot arrangement exclusive between different colors reducesdot overlapping and excessive paper white exposure. In particular,contrast increase in a local region in flat gradation data is reduced,and degradation of the granularity due to overlapping dots is reduced.In addition, color cloud due to overlapping between dots of differentink colors is reduced, and the color development characteristic isimproved at a dark part (highly dense part), in particular.

However, for example, when the gradation data abruptly changes in theprocessing target region, it is not preferable to arrange dots ofdifferent colors exclusively from each other. For example, in thegradation data illustrated in FIGS. 28A, 29A, 30A, and 31A, a line ofthe four colors of C, M, Y, and K in mixture is formed on the secondcolumn from the left. However, in dot arrangement using the evaluationvalue E′ after the exclusion control processing illustrated in FIG. 32B,dots of K and C do not overlap with each other, and an exclusivearrangement is achieved. Thus, no dot overlaps with pixels on which theline of the four colors in mixture should be formed, and the pixels arepotentially not perceived as a line of a mixed color. Alternatively,coloring other than the mixed color potentially locally occurs.

However, in the dot arrangement based on the evaluation value E″ afterthe exclusion control processing illustrated in FIG. 29H, dots of C inkand K ink overlap with each other on a CMYK mixed line. Similarly, dotsof M ink and Y ink illustrated in FIGS. 30H and 31H are arranged on aCMYK mixed line. In other words, the dot arrangement in which exclusionis prevented to allow overlapping in a region where the gradation dataabruptly changes is obtained by arranging dots in accordance with theevaluation value E″ after the exclusion control processing. In addition,in this case, in the dot arrangement based on the evaluation value E″after the exclusion control processing, arrangement exclusive betweendifferent colors is obtained in a uniform region on the first columnfrom the right on the gradation data.

However, in the dot arrangement based on the evaluation value E beforethe exclusion processing illustrated in FIG. 29C, dots of K ink and Cink overlap with each other, and the arrangement is not exclusivebetween different colors. In the present embodiment, when the gradationdata I is flat, the accumulative gradation data S is flat. In this case,the absolute values of the evaluation values E and E″ change, but therelative magnitude relation therebetween does not change. Thus, theorder of dot arrangement does not change between the evaluation valuesE′ and E″ in a flat gradation data region. Accordingly, a dotarrangement in which dots of different colors are exclusively arrangedcan be obtained in a flat gradation data region when the exclusioncontrol processing using the accumulative gradation data S is performed.

In other words, in the present embodiment, dot history information isreferred to, and evaluation values are corrected so that a dot isunlikely to be arranged at a pixel at which another dot is arranged.Then, the dot arrangement evaluation values are further corrected toreduce the above-described correction based on the contrast of thegradation data. Through such processing, in a region in which thecontrast is low, inter-color exclusion target dots can be arranged indispersion based on the magnitude of the threshold matrix, therebyimproving the granularity. In a region in which the contrast is high,dot exclusion can be prevented based on the magnitude of the gradationdata, thereby improving the reproducibility of shapes on image data.

Modifications of Fourth and Fifth Embodiments

In the above-described embodiments, the evaluation value E is describedas a real number but may be calculated as an 8-bit integer.

The degree of exclusion in a flat region may be adjusted by multiplyingthe exclusion control value H and the accumulative gradation data S by acoefficient of 0 to 1. In this case, when the coefficient is “1”,results same as those of the above-described embodiments can beobtained. As the coefficient is smaller, the exclusion processing in aflat region is reduced, and the number of dots at which overlappingbetween colors occurs increases. When the coefficient is “0”, theexclusion processing is not performed for a uniform region. For example,the coefficients for the exclusion value H and the gradation data S maybe determined in accordance with misregistration (relative positionalshift) between colors assumed for a printer. Specifically, robustnessagainst the misregistration between colors can be improved by performingadjustment such as decrease of the coefficients in a region in which therelative positional shift between colors is large and color unevennessis noticeable.

The above-described misregistration is hardly sensed at a highlight partin which the number of dots is small, and thus the above-describedcoefficients may be controlled in accordance with the luminance of aninput image.

In addition, an exclusion target color may be limited. Specifically, theabove-described processing may be performed only for the exclusiontarget color, and normal dither processing may be performed for a nonexclusion target color (or the above-described coefficients are set tobe zero for the non exclusion target color).

In the fifth embodiment, the target dot number is equal to or smallerthan 16 in a 4x4-pixel unit region. However, when multiple nozzle arraysare provided or when identical ink can be discharged over each pixel byscanning an identical region multiple times, dots in a number exceeding16 are arranged in the region in some cases. For example, when ink canbe discharged at each pixel up to twice, the gradation data and I_maxmay be determined so that I/I_max in the gradation data is normalized to0 to 2. Then, th/th_max may be normalized to 0 to 1, and the number ofdots to be arranged in the region may be calculated with the gradationdata and the threshold thus normalized. Specifically, first, dots may bearranged one by one at a pixel for which I/I_max is equal to or largerthan one, and then one may be subtracted from I/I_max of a pixel atwhich a dot is arranged, and any remaining dot may be arranged. In thiscase, history information 1111 may hold the number of arranged dots,which is 0 to 2, for each pixel.

The printing head can form dots at different discharge amounts in somecases. For example, processing can be performed similarly when large,intermediate, and small dots are formed. For example, the small dot maybe arranged at a pixel at which one dot is arranged, the intermediatedot may be arranged at a pixel at which two dots are arranged, and thelarge dot may be arranged at a pixel at which three dots are arranged.

In the fourth and fifth embodiments, the threshold matrix different foreach ink color is used for the dot number calculation and the dotarrangement order acquisition. However, an identical threshold matrixmay be used for all ink colors or multiple ink colors. Particularly in aflat region, an accumulative dot pattern of all ink colors is a patterngenerated based on a single threshold matrix, and thus dot dispersityimproves. In addition, when exclusion processing is performed withweighted dots of different colors, the dots can be arranged in the orderof dot luminance including dot overlapping. Accordingly, the contrastbetween the dots can be reduced, thereby achieving further improvementof the granularity.

In the fourth and fifth embodiments, the exclusion control value H andthe accumulative gradation data S are calculated with reference to dotarrangement of all colors processed before the processing target color.However, exclusion control may be performed only between some of thecolors. In this case, the exclusion control value H and the accumulativegradation data S may be generated only from dot patterns of specifiedcolors by means for specifying a reference color. In addition, theaccumulative gradation data S of an interest color may be calculated bysumming the gradation data of the interest color.

Other Embodiments

In the above-described first to fifth embodiments, all image processingillustrated in FIG. 3 is performed by the image processing apparatus(host PC) 200 illustrated in FIG. 2 , but the present invention is notlimited to such a configuration. Part of the image processingillustrated in FIG. 3 may be performed by the host PC, and the remainingprocessing may be performed by the printing apparatus 100, or allprocessing may be performed by the printing apparatus 100.Alternatively, a process up to halfway through the quantizationprocessing described with reference to FIGS. 5 and 10 may be performedby the host PC, and the remaining process may be performed by theprinting apparatus. In any case, a device configured to perform thequantization processing according to the present invention is the imageprocessing apparatus according to the present invention. When thequantization processing is performed by the host apparatus and theprinting apparatus in cooperation, this entire print system is the imageprocessing apparatus according to the present invention.

Embodiment(s) of the present invention can also be realized by acomputer of a system or apparatus that reads out and executes computerexecutable instructions (e.g., one or more programs) recorded on astorage medium (which may also be referred to more fully as a‘non-transitory computer-readable storage medium’) to perform thefunctions of one or more of the above-described embodiment(s) and/orthat includes one or more circuits (e.g., application specificintegrated circuit (ASIC)) for performing the functions of one or moreof the above-described embodiment(s), and by a method performed by thecomputer of the system or apparatus by, for example, reading out andexecuting the computer executable instructions from the storage mediumto perform the functions of one or more of the above-describedembodiment(s) and/or controlling the one or more circuits to perform thefunctions of one or more of the above-described embodiment(s). Thecomputer may comprise one or more processors (e.g., central processingunit (CPU), micro processing unit (MPU)) and may include a network ofseparate computers or separate processors to read out and execute thecomputer executable instructions. The computer executable instructionsmay be provided to the computer, for example, from a network or thestorage medium. The storage medium may include, for example, one or moreof a hard disk, a random-access the memory (RAM), a read only the memory(ROM), a storage of distributed computing systems, an optical disk (suchas a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc(BD)™), a flash memory device, a memory card, and the like.

The present invention can provide quantization processing that canreduce color development defect due to dot overlapping and can output animage with reduced granularity and excellent sharpness when the image isprinted by using multiple kinds of colorants.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Applications No.2018-229551 filed Dec. 7, 2018, and No. 2019-013881 filed Jan. 30, 2019,which are hereby incorporated by reference wherein in their entirety.

What is claimed is:
 1. An image processing apparatus that determines dotarrangement for each of a plurality of kinds of colorants based on animage for the colorant, the image processing apparatus comprising: anacquisition unit configured to acquire colorant data corresponding to aplurality of kinds of colorants; a determination unit configured todetermine dot arrangement for first dots and second dots eachcorresponding to a first colorant data and a second colorant data amongthe plurality of kinds of colorants at an interest pixel, wherein (i) adegree of exclusion of the first dots and the second dots at a regionincluding the interest pixel in a case where a dot is arranged at theinterest pixel and contrast of the region is a first contrast is lowerthan (ii) a degree of exclusion of the first dots and the second dots ata region including the interest pixel in a case where a dot is arrangedat the interest pixel and the contrast the region is a second contrastlower than the first contrast.
 2. The image processing apparatusaccording to claim 1, wherein the determination unit does not performdot exclusion for the second colorant data at the interest pixel in acase where the first colorant data at the interest pixel indicates thatno dot is arranged at the interest pixel.
 3. The image processingapparatus according to claim 1, wherein the determination unit furtherincludes: a first correction unit configured to correct a threshold forthe interest pixel based on the first colorant data at the interestpixel, and a second correction unit configured to further correct thethreshold corrected by the first correction unit, in accordance with thepixel value of the first colorant data at the interest pixel and a pixelvalue of the first colorant data at a pixel adjacent to the interestpixel, and a quantizing unit configured to quantize the pixel value ofthe second colorant data at the interest pixel by using the thresholdcorrected by the second correction unit.
 4. The image processingapparatus according to claim 3, wherein the first correction unit doesnot correct the threshold in a case where the first colorant data at theinterest pixel indicates that no dot is arranged at the interest pixel.5. The image processing apparatus according to claim 3, wherein thefirst correction unit corrects the threshold for the interest pixel bysubtracting the pixel value of the first colorant data at the interestpixel from the threshold for the interest pixel.
 6. An image processingmethod for determining dot arrangement for each of a plurality of kindsof colorants based on an image for the colorant, the image processingmethod comprising: acquiring colorant data corresponding to a pluralityof kinds of colorants; and determining dot arrangement for first dotsand second dots each corresponding to a first colorant data and a secondcolorant data among the plurality of kinds of colorants at an interestpixel, wherein (i) a degree of exclusion of the first dots and thesecond dots at a region including the interest pixel in a case where adot is arranged at the interest pixel and contrast of the region is afirst contrast is lower than (ii) a degree of exclusion of the firstdots and the second dots at a region including the interest pixel in acase where a dot is arranged at the interest pixel and the contrast theregion is a second contrast lower than the first contrast.
 7. Anon-transitory computer readable storage medium storing a program forcausing a computer to function as each unit of an image processingapparatus that determines dot arrangement for each of a plurality ofkinds of colorants based on an image for the colorant, the imageprocessing apparatus comprising: an acquisition unit configured toacquire colorant data corresponding to a plurality of kinds ofcolorants; and a determination unit configured to determine dotarrangement for first dots and second dots each corresponding to a firstcolorant data and a second colorant data among the plurality of kinds ofcolorants at an interest pixel, wherein (i) a degree of exclusion of thefirst dots and the second dots at a region including the interest pixelin a case where a dot is arranged at the interest pixel and contrast ofthe region is a first contrast is lower than (ii) a degree of exclusionof the first dots and the second dots at a region including the interestpixel in a case where a dot is arranged at the interest pixel and thecontrast the region is a second contrast lower than the first contrast.